Add openExternal IPC handler and integrate with UI components

- Introduce `openExternal` IPC handler in `electron/main.ts` for opening external links via the default browser or native app.
- Update typings in `electron.d.ts` and `preload.ts` to include `openExternal`.
- Refactor `ComposerRightBar` and `LeftBar` components to leverage `electron.openExternal` for external link actions (e.g., Facebook, Discord).
- Add offline mode logic to filter navigation options in `ComposerRightBar` and `LeftBar`.
- Enhance footer bar to display book titles when chapters are unavailable.

electron/preload.ts

@@ -35,6 +35,9 @@ contextBridge.exposeInMainWorld('electron', {
   // Database initialization (shortcut for convenience)
   dbInitialize: (userId: string, encryptionKey: string) => ipcRenderer.invoke('db-initialize', userId, encryptionKey),
 
+  // Open external links (browser/native app)
+  openExternal: (url: string) => ipcRenderer.invoke('open-external', url),
+
   // Offline mode management
   offlinePinSet: (pin: string) => ipcRenderer.invoke('offline:pin:set', { pin }),
   offlinePinVerify: (pin: string) => ipcRenderer.invoke('offline:pin:verify', { pin }),