Add an import plugin system, with the Overpass POI query as its first plugin

A plugin is a directory under plugins/ with a plugin.json manifest
({"label": "...", "command": "./run.sh"}) plus a script that reads
--south/--west/--north/--east and prints a GeoJSON FeatureCollection
to stdout (lib/maptool/plugins.rb) — that's the entire interface a
plugin needs to know about this app. The sidebar's new import icon
(logged in only, GET /api/plugins + POST /api/plugins/:name/run) lists
whatever's found and runs one for the current map viewport, saving its
output as a new .geojson file the same way an upload would be.

The point: a scraper for a site whose ToS forbids that (Park4Night,
iOverlander, Freecampsites) doesn't need to live in this public repo
to work here — its own directory just goes under plugins/ locally and
is never git add'ed, no special-casing needed anywhere.

plugins/overpass-pois wraps the existing rake overpass:pois query as
the first, in-repo, reference example of the contract.

Also fills in four modules (auth.js, locate.js, measure.js, notes.js)
missing from layout.erb's cache-busting importmap alongside adding
plugins.js's own entry — a pre-existing gap that meant they could keep
serving a stale cached copy across a deploy despite the importmap
existing specifically to prevent that.

public/js/plugins.js

@@ -0,0 +1,96 @@
+import { PLUGIN_ICON } from "/js/icons.js";
+import { notifyGpxStoreChanged } from "/js/panels.js";
+
+// A sidebar flyout listing whatever import plugins are on disk (see
+// lib/maptool/plugins.rb for the contract) — each one just a "Run for
+// this area" button using the current map viewport as the bbox, so there's
+// no separate area-picker to build: pan/zoom to where you want, click Run.
+// Only ever mounted when logged in (see map.js) — same "hide the
+// write-only affordance" story as NotesControl, since the only thing this
+// panel does is a login-gated write.
+export class PluginsControl {
+  constructor(map) {
+    this._map = map;
+  }
+
+  mount(sidebar) {
+    this._container = document.createElement("div");
+    this._container.className = "sidebar-item";
+    this._container.innerHTML = `
+      <button type="button" class="sidebar-icon" title="Import plugins" aria-label="Import plugins">${PLUGIN_ICON}</button>
+      <div class="sidebar-flyout" hidden>
+        <p class="plugins-panel__hint">Runs for whatever area is currently on screen.</p>
+        <ul class="plugins-panel__list"></ul>
+      </div>
+    `;
+    this._icon = this._container.querySelector(".sidebar-icon");
+    this._flyout = this._container.querySelector(".sidebar-flyout");
+    this._list = this._container.querySelector(".plugins-panel__list");
+
+    this._icon.addEventListener("click", () => {
+      const opening = this._flyout.hidden;
+      this._flyout.hidden = !opening;
+      this._icon.classList.toggle("active", opening);
+      if (opening) this._loadPlugins();
+    });
+
+    sidebar.appendChild(this._container);
+  }
+
+  async _loadPlugins() {
+    this._list.innerHTML = `<li class="plugins-panel__empty">Loading…</li>`;
+    let plugins;
+    try {
+      plugins = await (await fetch("/api/plugins")).json();
+    } catch (err) {
+      this._list.innerHTML = `<li class="plugins-panel__empty">Could not load plugins</li>`;
+      return;
+    }
+
+    if (plugins.length === 0) {
+      this._list.innerHTML = `<li class="plugins-panel__empty">No plugins installed — see docs/development.adoc</li>`;
+      return;
+    }
+    this._list.innerHTML = "";
+    plugins.forEach((plugin) => this._list.appendChild(this._renderItem(plugin)));
+  }
+
+  _renderItem(plugin) {
+    const li = document.createElement("li");
+    li.className = "plugins-panel__item";
+    li.innerHTML = `
+      <span class="plugins-panel__label"></span>
+      <button type="button" class="plugins-panel__run">Run for this area</button>
+      <span class="plugins-panel__status"></span>
+    `;
+    li.querySelector(".plugins-panel__label").textContent = plugin.label;
+
+    const button = li.querySelector(".plugins-panel__run");
+    const status = li.querySelector(".plugins-panel__status");
+    button.addEventListener("click", () => this._run(plugin, button, status));
+    return li;
+  }
+
+  async _run(plugin, button, status) {
+    button.disabled = true;
+    status.textContent = "Running…";
+    try {
+      const bounds = this._map.getBounds();
+      const body = new URLSearchParams({
+        south: bounds.getSouth(),
+        west: bounds.getWest(),
+        north: bounds.getNorth(),
+        east: bounds.getEast(),
+      });
+      const response = await fetch(`/api/plugins/${encodeURIComponent(plugin.name)}/run`, { method: "POST", body });
+      if (!response.ok) throw new Error(`run failed: ${response.status}`);
+      const { path } = await response.json();
+      notifyGpxStoreChanged(path);
+      status.textContent = "Done";
+    } catch (err) {
+      status.textContent = "Failed — see server log";
+    } finally {
+      button.disabled = false;
+    }
+  }
+}