diff --git a/frontend/static/bookmarks.js b/frontend/static/bookmarks.js
new file mode 100644
index 0000000..361adfa
--- /dev/null
+++ b/frontend/static/bookmarks.js
@@ -0,0 +1,182 @@
+// Bookmarked locations: the single source of truth for "places you've saved".
+// Logged out, this is pure localStorage. Logged in, the server is authoritative
+// and this module mirrors it into localStorage so the list still renders
+// instantly (and offline) on the next load. Every consumer (the map page's
+// star toggle + saved-locations dropdown, the alerts page's Saved locations
+// section) reads through list()/subscribe() and never touches storage or the
+// API directly.
+import { apiFetch, onAuthChange, uv } from "./account.js";
+
+const LS_KEY = "thermograph:bookmarks";
+const DISMISS_KEY = "thermograph:bookmarks:import-dismissed";
+const MAX_LOCAL = 200;
+
+// A stable local key for a spot: rounded to ~4dp (~11m), so re-picking
+// "the same place" a pixel off the first pin still matches.
+export function cellId(lat, lon) {
+  return `${lat.toFixed(4)},${lon.toFixed(4)}`;
+}
+
+function readLocal() {
+  try {
+    const arr = JSON.parse(localStorage.getItem(LS_KEY));
+    return Array.isArray(arr)
+      ? arr.filter((b) => b && typeof b.lat === "number" && typeof b.lon === "number")
+      : [];
+  } catch (e) { return []; }
+}
+function writeLocal(rows) {
+  try { localStorage.setItem(LS_KEY, JSON.stringify(rows.slice(0, MAX_LOCAL))); } catch (e) { /* private mode, quota, etc. — degrade quietly */ }
+}
+
+let cache = readLocal();     // what every consumer renders
+let pending = [];            // local-only bookmarks a signed-in user hasn't imported yet
+let signedIn = false;
+const subs = [];
+
+function notify() { subs.forEach((cb) => { try { cb(cache.slice()); } catch (e) {} }); }
+
+/** Subscribe to bookmark-list changes. Returns an unsubscribe function. */
+export function subscribe(cb) {
+  subs.push(cb);
+  return () => { const i = subs.indexOf(cb); if (i > -1) subs.splice(i, 1); };
+}
+
+export function list() { return cache.slice(); }
+export function find(lat, lon) {
+  const id = cellId(lat, lon);
+  return cache.find((b) => cellId(b.lat, b.lon) === id) || null;
+}
+export function isBookmarked(lat, lon) { return !!find(lat, lon); }
+export function pendingImportCount() { return pending.length; }
+
+export function importDismissed() {
+  try { return localStorage.getItem(DISMISS_KEY) === "1"; } catch (e) { return false; }
+}
+export function dismissImportPrompt() {
+  try { localStorage.setItem(DISMISS_KEY, "1"); } catch (e) {}
+  pending = [];
+  notify();
+}
+
+function fromServerRow(b) {
+  return {
+    id: b.id != null ? String(b.id) : (b.cell_id || cellId(b.lat, b.lon)),
+    cell_id: b.cell_id || null,
+    lat: b.lat, lon: b.lon,
+    label: b.label || null,
+    created_at: b.created_at || Date.now() / 1000,
+  };
+}
+
+// Returns the server's list, or null on 401 (signed out) / offline / error —
+// callers treat null as "can't say, fall back to local" rather than "empty".
+async function fetchServerList() {
+  try {
+    const res = await apiFetch(uv("bookmarks"));
+    if (res.status === 401) { signedIn = false; return null; }
+    if (!res.ok) return null;
+    signedIn = true;
+    const data = await res.json();
+    return Array.isArray(data.bookmarks) ? data.bookmarks : [];
+  } catch (e) { return null; }
+}
+
+// Reconciles cache with the server, diffing against whatever was in
+// localStorage right before this call so any local-only spots can be offered
+// up for import instead of silently vanishing behind the server's list.
+async function syncFromServer(preSyncLocal) {
+  const rows = await fetchServerList();
+  if (rows == null) return;        // signed out or offline: leave cache as-is
+  const serverList = rows.map(fromServerRow);
+  const serverIds = new Set(serverList.map((b) => cellId(b.lat, b.lon)));
+  const localOnly = preSyncLocal.filter((b) => !serverIds.has(cellId(b.lat, b.lon)));
+  cache = serverList;
+  writeLocal(cache);
+  pending = importDismissed() ? [] : localOnly;
+  notify();
+}
+
+/** Save (or re-save with a new label) the given spot. Upserts either way. */
+export async function add(lat, lon, label) {
+  const id = cellId(lat, lon);
+  if (signedIn) {
+    try {
+      const res = await apiFetch(uv("bookmarks"), { method: "POST", json: { lat, lon, label: label || undefined } });
+      if (res.ok) {
+        const rows = await fetchServerList();
+        if (rows != null) { cache = rows.map(fromServerRow); writeLocal(cache); notify(); return find(lat, lon); }
+      }
+    } catch (e) { /* fall through so the star still visibly sticks, even offline */ }
+  }
+  let b = cache.find((x) => cellId(x.lat, x.lon) === id);
+  if (b) { if (label) b.label = label; }
+  else {
+    b = { id, lat, lon, label: label || null, created_at: Date.now() / 1000 };
+    cache = [b, ...cache].slice(0, MAX_LOCAL);
+  }
+  writeLocal(cache);
+  notify();
+  return b;
+}
+
+export async function remove(id) {
+  if (signedIn) {
+    try { await apiFetch(uv(`bookmarks/${encodeURIComponent(id)}`), { method: "DELETE" }); } catch (e) {}
+  }
+  cache = cache.filter((b) => b.id !== id);
+  writeLocal(cache);
+  notify();
+}
+
+export async function rename(id, label) {
+  if (signedIn) {
+    try { await apiFetch(uv(`bookmarks/${encodeURIComponent(id)}`), { method: "PATCH", json: { label } }); }
+    catch (e) { /* still apply locally so the edit shows up */ }
+  }
+  const b = cache.find((x) => x.id === id);
+  if (b) { b.label = label; writeLocal(cache); notify(); }
+}
+
+async function runImport(items) {
+  if (!items.length) return { imported: 0, skipped: 0 };
+  const res = await apiFetch(uv("bookmarks/import"), { method: "POST", json: { items } });
+  if (!res.ok) throw new Error(`Import failed (${res.status}).`);
+  const data = await res.json();
+  if (Array.isArray(data.bookmarks)) { cache = data.bookmarks.map(fromServerRow); writeLocal(cache); }
+  pending = [];
+  notify();
+  return data;
+}
+
+/** Import the local-only bookmarks the login prompt flagged. */
+export function importPending() {
+  return runImport(pending.map((b) => ({ lat: b.lat, lon: b.lon, label: b.label })));
+}
+
+/** Import everything currently in localStorage — the explicit "Import from
+ * this device" button on the alerts page, reachable even after the login
+ * prompt was dismissed. */
+export function importAllLocal() {
+  return runImport(readLocal().map((b) => ({ lat: b.lat, lon: b.lon, label: b.label })));
+}
+
+// --- boot + auth transitions -------------------------------------------------
+(async function boot() {
+  const local = readLocal();
+  cache = local;
+  notify();                       // render local state immediately, don't wait on the network
+  await syncFromServer(local);
+})();
+
+onAuthChange(async (user) => {
+  if (user) {
+    await syncFromServer(readLocal());
+  } else {
+    // Logged out: fall back to local bookmarks without wiping them.
+    signedIn = false;
+    pending = [];
+    cache = readLocal();
+    notify();
+  }
+});