feat: sync notification badge + clarify cloud storage support

Notification system:
- When sync applies data (file folder, browser account, or sync code),
  ss_sync_notification is written to local storage with the source.
- Service worker catches it via storage.onChanged, shows a purple 'SYN'
  badge on the extension icon that persists until Options is opened, and
  fires a desktop notification ('Settings updated via sync folder — open
  Options to review').
- Clicking the desktop notification opens the Options page directly.
- On service worker wake, SYN badge is restored if the notification was
  not yet dismissed.
- Opening Options clears ss_sync_notification, resets the badge, and
  sends a sync:notification-seen message to the service worker.
- Added 'notifications' permission to both manifests.

Cloud storage clarity:
- Options page now explicitly lists that the folder sync works with any
  cloud storage that has a desktop sync client: Dropbox, OneDrive, Google
  Drive, iCloud Drive, Box, pCloud, Nextcloud, Synology Drive, etc.

https://claude.ai/code/session_01TKpSR9M8JgHLXCp5CeDsQP

manifest.json

@@ -6,7 +6,8 @@
   "permissions": [
     "storage",
     "activeTab",
-    "scripting"
+    "scripting",
+    "notifications"
   ],
   "host_permissions": [
     "https://claude.ai/*",