Vue migration: Build and smoke-test in Firefox

[exhuma]

Part of #19.\n\nFinal verification step:\n\n- Run npm run dist — build must succeed with no errors\n- Run npm run browser — load the extension in Firefox\n- Verify sidebar opens and displays bookmarks\n- Verify tag filtering works (click tag → list filters, click again → removes filter)\n- Verify search works\n- Verify narrow mode (sidebar) and wide mode both render correctly\n- Verify the popup (page_action.html) still adds/removes bookmarks\n- Verify bookmarks are refreshed after adding/removing via popup