* Add browser import hint button to the browser toolbar
- Adds a dismissible "Import" button in the browser address bar that opens a popover with browser detection and one-click cookie import
- Persists the hint's dismissed state via `browserImportHintHidden` in `PersistedUIState` so the hint stays hidden after the user clicks "Hide Hint"
- Extracts platform-aware import source labels into `browser-cookie-import-sources.ts` and hint visibility logic into `browser-import-hint-visibility.ts` for isolated testing
* fix the onblur for the import popover
