Skip rebuilding offline/terrain data when nothing actually changed

public/js/offline_data.js

@@ -274,6 +274,23 @@ 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…";
+    try {
+      // false (not null/true — see offline_update_available? in
+      // pmtiles_builder.rb) means the just-saved selection matches what's
+      // already on disk byte-for-byte: same countries, nothing upstream
+      // changed. Rebuilding would just burn bandwidth/CPU to reproduce
+      // the same file, so skip it. null (never built, or can't tell)
+      // still falls through to building below.
+      const check = await (await fetch(this._checkUpdateUrl)).json();
+      if (check.update_available === false) {
+        this._refreshStatus();
+        return;
+      }
+    } catch {
+      // couldn't tell — fall through and build rather than silently skip
+    }
+
     this._statusEl.textContent = "Starting…";
     try {
       const response = await fetch(this._buildUrl, { method: "POST" });