fix: eliminate OS keychain prompt on startup by persisting pairing token in DB

The root cause was hydrate_extension_pairing_token accessing the
keychain when keychainAccessGranted was true in the DB.  Any keychain
access on macOS triggers the system prompt when the binary signature
changes after an update.

Architecture change: two-store model.
- The SQLite settings DB is now the primary store for the token.
  hydrate_extension_pairing_token reads from it exclusively -- it
  never touches the OS keychain.  No system prompt on startup.
- The OS keychain remains defence-in-depth: grant_keychain_access
  still writes the token there, but it is only reached from the
  explicit Grant Access button, so any system prompt is user-initiated.

DB helpers: load_pairing_token_from_settings / save_pairing_token_to_settings
hydrate_extension_pairing_token: reads from DB, skips keychain entirely
grant_keychain_access: syncs token to DB after keychain access
Frontend: extensionPairingToken included in partialize for auto-persist

src-tauri/src/settings.rs

@@ -298,6 +298,7 @@ fn default_settings() -> PersistedSettings {
         prevents_sleep_while_downloading: true,
         media_cookie_source: MediaCookieSource::None,
         site_logins: Vec::new(),
+        extension_pairing_token: String::new(),
         auto_check_updates: true,
         keychain_access_granted: false,
     }