Add a country-based region picker for offline data

Replaces "regenerate for the current viewport" with what Locus/OsmAnd-
style apps give you: tick which countries you want offline (a Natural
Earth-derived checklist, public domain, vendored), Apply rebuilds
offline.pmtiles/terrain.pmtiles to cover the whole selection at once.
Multiple regions can be offline simultaneously (e.g. Slovakia + Austria
for a trip that crosses the border) — the selection itself is the
source of truth (data/offline_regions.json), not a set of per-country
files, so "remove a country" and "refresh a country" are both just
"change the selection, then Apply."

Confirmed against go-pmtiles' own source (not just its docs) before
committing to this shape: `extract --region=<geojson>` treats a
FeatureCollection's features as their union already (no dissolve/union
math needed on our side — pmtiles/region.go's UnmarshalRegion just
concatenates every feature's rings), and `merge` hard-errors on any
overlapping tile id between inputs (merge.go's prepareInputs). That
second fact rules out "extract each country separately, merge them
all" — adjacent countries' region-extracts would share border tiles,
since extraction copies whole tiles rather than clipping vector
content to the polygon. The actual fix: extract per *source archive*
(the Protomaps build, each intersecting Mapterhorn chunk), each call
covering the whole current selection via one --region= file — the
terrain pipeline already extracted-then-merged per source this way, it
just needed a multi-country region instead of a single bbox.

public/js/map.js

@@ -516,7 +516,7 @@ if (isLoggedIn()) new PluginsControl(map).mount(sidebar);
 sidebar.insertAdjacentHTML("beforeend", `<div class="sidebar-group-label" title="Offline hub"></div>`);
 layerToggle.mount(sidebar);
 layerToggle.setActive("online");
-new OfflineDataControl(map).mount(sidebar);
+new OfflineDataControl().mount(sidebar);
 
 sidebar.insertAdjacentHTML("beforeend", `<div class="sidebar-group-label" title="Navigation"></div>`);
 gnssControl.mount(sidebar);

public/js/offline_data.js

@@ -1,6 +1,7 @@
 import { OFFLINE_DATA_ICON } from "/js/icons.js";
 import { notifySidePanelOpen, onlyOneSidePanelOpen } from "/js/panels.js";
 import { isLoggedIn } from "/js/auth.js";
+import { escapeHtml } from "/js/gpx.js";
 
 const POLL_INTERVAL_MS = 3000;
 
@@ -16,51 +17,106 @@ function formatElapsed(ms) {
   return minutes > 0 ? `${minutes}m ${seconds}s` : `${seconds}s`;
 }
 
-// "Building…" alone gave no way to tell a slow build (terrain.pmtiles can
-// take minutes — lib/maptool/pmtiles_builder.rb fetches tiles one at a
-// time) from a stuck one. Elapsed time alone answers "is it alive" for
-// both files; tile counts (only terrain.pmtiles reports these) give an
-// actual sense of how much is left.
+// "Building…" alone gave no way to tell a slow build from a stuck one —
+// elapsed time answers "is it alive" regardless of how long a given
+// selection takes.
 function formatRunning(status) {
   const elapsed = status.started_at ? ` (${formatElapsed(Date.now() - new Date(status.started_at).getTime())})` : "";
-  const tiles = status.tiles_total ? ` · ${status.tiles_done}/${status.tiles_total} tiles` : "";
-  return `Building…${elapsed}${tiles}`;
+  return `Building…${elapsed}`;
 }
 
-// One row of the Offline data panel — status text plus a "Regenerate for
-// current view" button, for either offline.pmtiles or terrain.pmtiles.
-// Both files behave identically from here (same status/build/build-status
-// endpoint shape, see lib/maptool/pmtiles_builder.rb), just against
-// different URLs/labels, so one class covers both rather than writing the
-// same fetch/poll logic twice.
+// The checklist of selectable countries (GET /api/offline-regions) plus
+// the "Apply changes" button that saves the ticked set (POST, same URL)
+// and hands it off — doesn't trigger a build itself, OfflineDataControl
+// does that across both files once the selection is saved.
+class RegionPicker {
+  constructor(onApply) {
+    this._onApply = onApply;
+  }
+
+  async mount(container) {
+    const section = document.createElement("div");
+    section.className = "offline-data-panel__regions";
+    section.innerHTML = `
+      <p class="offline-data-panel__regions-label">Regions</p>
+      <ul class="offline-data-panel__region-list"><li class="offline-data-panel__region-status">Loading…</li></ul>
+      <button type="button" class="offline-data-panel__apply" hidden>Apply changes</button>
+    `;
+    this._list = section.querySelector(".offline-data-panel__region-list");
+    this._applyButton = section.querySelector(".offline-data-panel__apply");
+    // Viewing/ticking works for anyone; only saving needs a login — same
+    // finer-grained gating measure.js/gpx.js use for their own
+    // write-only affordances, rather than hiding this whole control.
+    this._applyButton.hidden = !isLoggedIn();
+    this._applyButton.addEventListener("click", () => this._apply());
+    container.appendChild(section);
+
+    let data;
+    try {
+      data = await (await fetch("/api/offline-regions")).json();
+    } catch {
+      this._list.innerHTML = `<li class="offline-data-panel__region-status">Could not load the region list.</li>`;
+      return;
+    }
+
+    const selected = new Set(data.selected);
+    this._list.innerHTML = data.available
+      .map(
+        (region) => `
+        <li>
+          <label>
+            <input type="checkbox" value="${escapeHtml(region.code)}" ${selected.has(region.code) ? "checked" : ""}>
+            ${escapeHtml(region.name)}
+          </label>
+        </li>`,
+      )
+      .join("");
+  }
+
+  async _apply() {
+    const codes = [...this._list.querySelectorAll("input:checked")].map((el) => el.value);
+    const body = new URLSearchParams();
+    codes.forEach((code) => body.append("codes[]", code));
+
+    this._applyButton.disabled = true;
+    try {
+      const response = await fetch("/api/offline-regions", { method: "POST", body });
+      const data = await response.json().catch(() => ({}));
+      if (!response.ok) throw new Error(data.error || `could not save the selection: ${response.status}`);
+    } catch (err) {
+      window.alert(err.message);
+      return;
+    } finally {
+      this._applyButton.disabled = false;
+    }
+
+    this._onApply();
+  }
+}
+
+// One row of the Offline data panel — just a status readout for either
+// offline.pmtiles or terrain.pmtiles, both driven by the same selection
+// (RegionPicker above). Both files behave identically from here (same
+// status/build/build-status endpoint shape, see
+// lib/maptool/pmtiles_builder.rb), just against different URLs/labels, so
+// one class covers both rather than writing the same fetch/poll logic
+// twice.
 class DataFileRow {
-  constructor(map, { label, statusUrl, buildUrl, buildStatusUrl }) {
-    this._map = map;
+  constructor({ label, statusUrl, buildUrl, buildStatusUrl }) {
     this._label = label;
     this._statusUrl = statusUrl;
     this._buildUrl = buildUrl;
     this._buildStatusUrl = buildStatusUrl;
-    this._exists = false;
   }
 
   mount(container) {
     const row = document.createElement("div");
     row.className = "offline-data-panel__row";
     row.innerHTML = `
-      <div class="offline-data-panel__row-header">
-        <strong>${this._label}</strong>
-        <button type="button" class="offline-data-panel__build">Regenerate for current view</button>
-      </div>
+      <strong>${this._label}</strong>
       <p class="offline-data-panel__status">Checking…</p>
     `;
-    this._buildButton = row.querySelector(".offline-data-panel__build");
     this._statusEl = row.querySelector(".offline-data-panel__status");
-    // Viewing status works for anyone; only triggering a build needs a
-    // login — same finer-grained gating measure.js/gpx.js use for their
-    // own write-only affordances, rather than hiding this whole control.
-    this._buildButton.hidden = !isLoggedIn();
-    this._buildButton.addEventListener("click", () => this._build());
-
     container.appendChild(row);
     this._refreshStatus();
   }
@@ -73,34 +129,21 @@ class DataFileRow {
       this._statusEl.textContent = "Could not check status.";
       return;
     }
-    this._exists = data.available;
     this._statusEl.textContent = data.available
       ? `${formatBytes(data.size)} · updated ${new Date(data.mtime).toLocaleString([], { dateStyle: "short", timeStyle: "short" })}`
       : "Not generated yet.";
   }
 
-  async _build() {
-    if (this._exists && !window.confirm(`Replace the existing ${this._label.toLowerCase()} with a fresh build for the current view?`)) {
-      return;
-    }
-
-    const bounds = this._map.getBounds();
-    const body = new URLSearchParams({
-      south: bounds.getSouth(),
-      west: bounds.getWest(),
-      north: bounds.getNorth(),
-      east: bounds.getEast(),
-    });
-
-    this._buildButton.disabled = true;
+  // Called by OfflineDataControl once RegionPicker's Apply has saved a
+  // new selection — not tied to a button of this row's own.
+  async triggerBuild() {
     this._statusEl.textContent = "Starting…";
     try {
-      const response = await fetch(this._buildUrl, { method: "POST", body });
+      const response = await fetch(this._buildUrl, { method: "POST" });
       const data = await response.json().catch(() => ({}));
       if (!response.ok) throw new Error(data.error || `build failed: ${response.status}`);
     } catch (err) {
       this._statusEl.textContent = err.message;
-      this._buildButton.disabled = false;
       return;
     }
 
@@ -126,7 +169,6 @@ class DataFileRow {
         return;
       }
 
-      this._buildButton.disabled = false;
       if (status.state === "error") {
         this._statusEl.textContent = `Build failed: ${status.message}`;
       } else {
@@ -137,27 +179,29 @@ class DataFileRow {
 }
 
 // A sidebar panel wrapping the manual go-pmtiles workflow
-// docs/development.adoc documents — "Regenerate for current view" on
-// either file shells out to it server-side instead of an admin running it
-// by hand (see lib/maptool/pmtiles_builder.rb). No area-picker: the
-// current map viewport is the region, same convention the Overpass POI
-// plugin already uses for "give me a bbox."
+// docs/development.adoc documents — a region picker (which countries
+// should be available offline) plus, on "Apply changes", a rebuild of
+// offline.pmtiles/terrain.pmtiles covering the whole current selection
+// (see lib/maptool/pmtiles_builder.rb — there's no standalone per-country
+// file to individually delete; "remove a country" and "refresh a
+// country" are both just "change the selection, then Apply").
 export class OfflineDataControl {
-  constructor(map) {
+  constructor() {
     this._rows = [
-      new DataFileRow(map, {
+      new DataFileRow({
         label: "Offline map data",
         statusUrl: "/api/offline-tiles",
         buildUrl: "/api/offline-tiles/build",
         buildStatusUrl: "/api/offline-tiles/build-status",
       }),
-      new DataFileRow(map, {
+      new DataFileRow({
         label: "Offline terrain data",
         statusUrl: "/api/terrain-tiles",
         buildUrl: "/api/terrain-tiles/build",
         buildStatusUrl: "/api/terrain-tiles/build-status",
       }),
     ];
+    this._regionPicker = new RegionPicker(() => this._rows.forEach((row) => row.triggerBuild()));
   }
 
   mount(sidebar) {
@@ -171,6 +215,7 @@ export class OfflineDataControl {
     this._panel.querySelector(".basemap-panel__close").addEventListener("click", () => this._togglePanel(false));
 
     const body = this._panel.querySelector(".offline-data-panel__body");
+    this._regionPicker.mount(body);
     this._rows.forEach((row) => row.mount(body));
 
     onlyOneSidePanelOpen(this._panel, () => this._icon.classList.remove("active"));