fix: prevent deleted toolbar bookmarks from being reverted by auto-sync (#6)

* fix: prevent deleted toolbar bookmarks from being reverted by auto-sync

Race condition: when a user deletes a bookmark while a sync is running,
performSync's storeTombstones() overwrites the tombstone created by the
onRemoved listener, causing the follow-up sync to re-add the bookmark
from cloud.

Fix: re-read current tombstones from storage before writing to preserve
any tombstones added concurrently during the sync.

* test: add integration test calling real performSync with mocked browser APIs

Add __test__ exports (VITEST-only, tree-shaken in production) to
background/index.js so integration tests can import and call the real
performSync, addTombstone, categorizeCloudBookmarks, etc.

New test file: toolbar-delete-race-integration.test.js (10 tests)
- Calls the REAL performSync with fully mocked browser.* and fetch APIs
- Simulates the tombstone race condition: delays cloud GET, fires
  onRemoved mid-sync, verifies tombstone is preserved after sync
- Verifies deleted bookmark is NOT re-added from cloud
- Tests tombstone merge, categorization filtering, concurrent sync
  guard, and state management
- All 10 tests pass; 527 total passing (was 517)

* Update apps/extension/__tests__/toolbar-delete-revert.test.js

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* chore: add screenshots/ to .gitignore

* chore(release): v0.8.27

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: Preshy

.gitignore

@@ -46,6 +46,9 @@ coverage/
 supabase/.branches/
 supabase/.temp/
 
+# Screenshots
+screenshots/
+
 # Misc
 *.local
 .cache/