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

manifest.json

@@ -1,6 +1,6 @@
 {
   "manifest_version": 3,
-  "name": "Shelf — Read Later",
+  "name": "Read it later",
   "version": "1.0.0",
   "description": "Save pages and links for later, and see them all as a sortable shelf every time you open a new tab.",
   "icons": {
@@ -9,13 +9,13 @@
     "48": "icons/icon48.png",
     "128": "icons/icon128.png"
   },
-  "permissions": ["storage", "contextMenus", "activeTab", "unlimitedStorage"],
+  "permissions": ["storage", "contextMenus", "activeTab", "unlimitedStorage", "alarms"],
   "background": {
     "service_worker": "background.js",
-    "scripts": ["background.js"]
+    "scripts": ["sync.js", "background.js"]
   },
   "action": {
-    "default_title": "Save this page to Shelf",
+    "default_title": "Save this page to Read it later",
     "default_icon": {
       "16": "icons/icon16.png",
       "32": "icons/icon32.png",