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

src/options/options.html

@@ -147,8 +147,9 @@
           Pick the same folder in each browser once. Changes are written to
           <code>silent-send-sync.json</code> automatically whenever settings change,
           and read back whenever this page is opened or regains focus.
-          Use a shared folder (Dropbox, OneDrive, iCloud Drive, or any network share)
-          for cross-computer sync too.
+          Works with <strong>any cloud storage that has a desktop sync client</strong> —
+          Dropbox, OneDrive, Google Drive, iCloud Drive, Box, pCloud, Nextcloud,
+          Synology Drive, or any network share. Just pick the cloud-synced folder.
         </p>
         <div class="bulk-actions" style="align-items:center">
           <button class="btn btn-primary" id="btnPickSyncFolder">Choose Sync Folder</button>