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/bindings/PersistedSettings.ts

@@ -8,4 +8,4 @@ import type { SettingsTab } from "./SettingsTab";
 import type { SiteLogin } from "./SiteLogin";
 import type { Theme } from "./Theme";
 
-export type PersistedSettings = { theme: Theme, baseDownloadFolder: string, categorySubfolders: { [key in string]: string }, categoryDirectoryOverrides: { [key in string]: string }, approvedDownloadRoots: Array<string>, maxConcurrentDownloads: number, globalSpeedLimit: string, isSidebarVisible: boolean, activeSettingsTab: SettingsTab, scheduler: SchedulerSettings, schedulerRunning: boolean, schedulerActiveDownloadIds: Array<string>, schedulerLastStartKey: string, schedulerLastStopKey: string, lastCustomSpeedLimitKiB: number, perServerConnections: number, maxAutomaticRetries: number, showNotifications: boolean, playCompletionSound: boolean, appFontSize: AppFontSize, listRowDensity: ListRowDensity, showDockBadge: boolean, showMenuBarIcon: boolean, proxyMode: ProxyMode, proxyHost: string, proxyPort: number, customUserAgent: string, askWhereToSaveEachFile: boolean, preventsSleepWhileDownloading: boolean, mediaCookieSource: MediaCookieSource, siteLogins: Array<SiteLogin>, autoCheckUpdates: boolean, keychainAccessGranted: boolean, };
+export type PersistedSettings = { theme: Theme, baseDownloadFolder: string, categorySubfolders: { [key in string]: string }, categoryDirectoryOverrides: { [key in string]: string }, approvedDownloadRoots: Array<string>, maxConcurrentDownloads: number, globalSpeedLimit: string, isSidebarVisible: boolean, activeSettingsTab: SettingsTab, scheduler: SchedulerSettings, schedulerRunning: boolean, schedulerActiveDownloadIds: Array<string>, schedulerLastStartKey: string, schedulerLastStopKey: string, lastCustomSpeedLimitKiB: number, perServerConnections: number, maxAutomaticRetries: number, showNotifications: boolean, playCompletionSound: boolean, appFontSize: AppFontSize, listRowDensity: ListRowDensity, showDockBadge: boolean, showMenuBarIcon: boolean, proxyMode: ProxyMode, proxyHost: string, proxyPort: number, customUserAgent: string, askWhereToSaveEachFile: boolean, preventsSleepWhileDownloading: boolean, mediaCookieSource: MediaCookieSource, siteLogins: Array<SiteLogin>, extensionPairingToken: string, autoCheckUpdates: boolean, keychainAccessGranted: boolean, };

src/store/useSettingsStore.ts

@@ -398,6 +398,7 @@ export const useSettingsStore = create<SettingsState>()(
         preventsSleepWhileDownloading: state.preventsSleepWhileDownloading,
         mediaCookieSource: state.mediaCookieSource,
         siteLogins: state.siteLogins,
+        extensionPairingToken: state.extensionPairingToken,
         keychainAccessGranted: state.keychainAccessGranted,
         autoCheckUpdates: state.autoCheckUpdates
       }),