feat: add reminders, auto-delete, cross-device sync, dark mode, and rediscovery spotlight

- Rename extension from Shelf to "Read it later"
- Add reminder settings: highlight and pin overdue unarchived items
- Add auto-delete with a hard safety rule requiring its period to exceed the reminder period
- Add cross-device sync via chrome.storage.sync (lightweight per-item metadata, no thumbnails)
- Add light/dark/system theme support
- Fix list-view Archive/Delete button alignment (card-perf wasn't filling the row)
- Add "Worth a second look" spotlight: proactively resurfaces old, forgotten items with automatic backoff so repeatedly-ignored items are shown less often, never permanently

background.js

@@ -1,8 +1,17 @@
-// Shelf — background service worker
+// Read it later — background service worker
 // Works under both the `chrome` namespace (Chrome/Edge) and Firefox's
 // chrome.* alias for WebExtensions.
 
+// Chrome loads only "service_worker" from the manifest (a Worker context, so
+// importScripts works); Firefox loads the "scripts" array instead, which
+// already includes sync.js ahead of this file in the same global scope.
+if (typeof importScripts === "function") {
+  importScripts("sync.js");
+}
+
 const STORAGE_KEY = "shelfItems";
+const PREFS_KEY = "shelfPrefs";
+const AUTO_DELETE_ALARM = "autoDeleteSweep";
 
 function domainOf(url) {
   try {
@@ -21,25 +30,51 @@ async function setItems(items) {
   await chrome.storage.local.set({ [STORAGE_KEY]: items });
 }
 
+async function getPrefs() {
+  const data = await chrome.storage.local.get(PREFS_KEY);
+  return data[PREFS_KEY] || {};
+}
+
+// Runs on an hourly alarm so overdue items get cleaned up even when the
+// shelf page is never opened. Silent — no one's necessarily watching, so
+// unlike the newtab-page sweep there's no toast/undo here; the newtab page
+// runs its own courtesy-toast sweep whenever it's open.
+async function sweepAutoDelete() {
+  const prefs = await getPrefs();
+  if (!prefs.autoDeleteEnabled) return;
+
+  const items = await getItems();
+  const remaining = items.filter((item) => !isOverdueForDeletion(item, prefs));
+  if (remaining.length === items.length) return;
+
+  const merged = typeof reconcileWithSync === "function" ? await reconcileWithSync(remaining) : remaining;
+  await setItems(merged);
+}
+
 async function addItem({ url, title, favIconUrl, image }) {
   if (!url) return;
-  const items = await getItems();
+  let items = await getItems();
 
   // De-dupe: if already saved, just bump it to the top and refresh metadata.
   const existingIndex = items.findIndex((i) => i.url === url);
+  const now = Date.now();
   const entry = {
-    id: existingIndex >= 0 ? items[existingIndex].id : `${Date.now()}-${Math.random().toString(36).slice(2, 8)}`,
+    id: existingIndex >= 0 ? items[existingIndex].id : `${now}-${Math.random().toString(36).slice(2, 8)}`,
     url,
     title: title || url,
     domain: domainOf(url),
     favicon: favIconUrl || "",
     image: image || (existingIndex >= 0 ? items[existingIndex].image : ""),
-    dateAdded: Date.now(),
+    dateAdded: now,
     archived: false,
+    updatedAt: now,
+    surfaceCount: existingIndex >= 0 ? items[existingIndex].surfaceCount || 0 : 0,
+    lastSurfacedAt: existingIndex >= 0 ? items[existingIndex].lastSurfacedAt || 0 : 0,
   };
 
   if (existingIndex >= 0) items.splice(existingIndex, 1);
   items.unshift(entry);
+  items = typeof reconcileWithSync === "function" ? await reconcileWithSync(items) : items;
   await setItems(items);
   return entry;
 }
@@ -111,3 +146,14 @@ chrome.contextMenus.onClicked.addListener(async (info, tab) => {
     if (tab && tab.id) flashBadge(tab.id, "✓");
   }
 });
+
+// --- Auto-delete: hourly sweep, independent of whether the shelf is open ---
+// chrome.alarms.create is idempotent (re-arming an existing alarm just
+// resets it), so it's safe to call unconditionally every time this script
+// runs rather than only from onInstalled.
+chrome.alarms.create(AUTO_DELETE_ALARM, { periodInMinutes: 60 });
+sweepAutoDelete();
+
+chrome.alarms.onAlarm.addListener((alarm) => {
+  if (alarm.name === AUTO_DELETE_ALARM) sweepAutoDelete();
+});