feat: about tab with update check/download (Gitea feed first, GitHub fallback)

- updater: dual-feed state machine, manual check/download/install IPC,
  changelog from Gitea releases API with GitHub fallback
- settings: new About tab (version, channel, auto-check toggle, progress)
- system.autoCheckUpdate setting gates the startup check

src/main/index.ts

@@ -5,7 +5,7 @@ import { registerIpc } from './ipc'
 import { killAllPtys } from './pty'
 import { applyStartupSystemSettings, loadSettings } from './settingsStore'
 import { initTray, markQuitting, onMainWindowClose } from './tray'
-import { configureAutoUpdater } from './updater'
+import { configureAutoUpdater, registerUpdateIpc } from './updater'
 import { applyWindowChrome } from './windowChrome'
 import { getThemeById } from '@shared/theme'