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/shared/contracts/channels.ts

@@ -15,6 +15,7 @@ export const ipcChannels = {
   setPatternFavorite: 'patterns:set-favorite',
   setTheme: 'settings:set-theme',
   setTerminalHeight: 'settings:set-terminal-height',
+  setNotificationsEnabled: 'settings:set-notifications-enabled',
   saveMcpServer: 'tooling:mcp:save',
   deleteMcpServer: 'tooling:mcp:delete',
   saveLspProfile: 'tooling:lsp:save',