feat: add desktop notifications for run completion

Show native OS notifications when a session run completes, fails, or
needs approval while the app window is unfocused. Clicking a
notification focuses the window and selects the relevant session.

Includes a toggle in Settings > Appearance to enable/disable
notifications (enabled by default).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/preload/index.ts

@@ -26,6 +26,7 @@ const api: ElectronApi = {
   setPatternFavorite: (input) => ipcRenderer.invoke(ipcChannels.setPatternFavorite, input),
   setTheme: (theme) => ipcRenderer.invoke(ipcChannels.setTheme, theme),
   setTerminalHeight: (input) => ipcRenderer.invoke(ipcChannels.setTerminalHeight, input),
+  setNotificationsEnabled: (enabled) => ipcRenderer.invoke(ipcChannels.setNotificationsEnabled, enabled),
   saveMcpServer: (input) => ipcRenderer.invoke(ipcChannels.saveMcpServer, input),
   deleteMcpServer: (serverId) => ipcRenderer.invoke(ipcChannels.deleteMcpServer, serverId),
   saveLspProfile: (input) => ipcRenderer.invoke(ipcChannels.saveLspProfile, input),