Truncate noisy build-failure output, fix the recurring terrain death

The web UI was showing a raw ~500k-character stdout/stderr dump on a
failed build — go-pmtiles' extract logs one progress line per chunk,
which is fine on a terminal (it overwrites itself) but not captured
non-interactively. command_output now keeps just the tail, and the
status line renders it with preserved line breaks in a scrollable box
instead of one unreadable blob.

Root cause of the actual failure (terrain build dying ~11 minutes in
with an HTTP/2 stream reset): go-pmtiles' default --overfetch=0.05
merged this multi-country planet.pmtiles extract into a single ~60GB
request with no way to resume after a dropped stream. --overfetch=0
splits it into many smaller requests instead.

public/js/offline_data.js

@@ -206,23 +206,31 @@ class DataFileRow {
     this._refreshStatus();
   }
 
+  // Single place that writes to _statusEl, so the error styling (pre-wrap,
+  // scrollable, red — see style.css) never stays stuck on once a build's
+  // failure message has scrolled past into a normal status line.
+  _setStatus(text, isError = false) {
+    this._statusEl.textContent = text;
+    this._statusEl.classList.toggle("offline-data-panel__status--error", isError);
+  }
+
   async _refreshStatus() {
     let data;
     try {
       data = await (await fetch(this._statusUrl)).json();
     } catch {
-      this._statusEl.textContent = "Could not check status.";
+      this._setStatus("Could not check status.");
       return;
     }
 
     if (!data.available) {
-      this._statusEl.textContent = "Not generated yet.";
+      this._setStatus("Not generated yet.");
       this._deleteButton.hidden = true;
       return;
     }
 
     const updated = new Date(data.mtime).toLocaleString([], { dateStyle: "short", timeStyle: "short" });
-    this._statusEl.textContent = `${formatBytes(data.size)} · updated ${updated}${this._formatMeta(data.meta)}`;
+    this._setStatus(`${formatBytes(data.size)} · updated ${updated}${this._formatMeta(data.meta)}`);
     this._deleteButton.hidden = !isLoggedIn();
     this._checkUpdate();
   }
@@ -274,7 +282,7 @@ class DataFileRow {
   // 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 = "Checking…";
+    this._setStatus("Checking…");
     try {
       // false (not null/true — see offline_update_available? in
       // pmtiles_builder.rb) means the just-saved selection matches what's
@@ -291,17 +299,17 @@ class DataFileRow {
       // couldn't tell — fall through and build rather than silently skip
     }
 
-    this._statusEl.textContent = "Starting…";
+    this._setStatus("Starting…");
     try {
       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._setStatus(err.message, true);
       return;
     }
 
-    this._statusEl.textContent = "Building…";
+    this._setStatus("Building…");
     this._poll();
   }
 
@@ -318,13 +326,13 @@ class DataFileRow {
       }
 
       if (status.state === "running") {
-        this._statusEl.textContent = formatRunning(status);
+        this._setStatus(formatRunning(status));
         this._poll();
         return;
       }
 
       if (status.state === "error") {
-        this._statusEl.textContent = `Build failed: ${status.message}`;
+        this._setStatus(`Build failed: ${status.message}`, true);
       } else {
         this._refreshStatus();
       }