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/settingsStore.ts

@@ -21,7 +21,8 @@ const DEFAULT_SYSTEM: SystemSettings = {
   launchAtLogin: false,
   preventSleep: false,
   globalShowHide: '',
-  closeAction: 'ask'
+  closeAction: 'ask',
+  autoCheckUpdate: true
 }
 
 const TERMINAL_KEYS = new Set(Object.keys(DEFAULT_SETTINGS.terminal) as (keyof TerminalSettings)[])
@@ -91,7 +92,8 @@ function deepMerge(raw: unknown): { settings: AppSettings; errors: string[] } {
         launchAtLogin: candidate.launchAtLogin === true,
         preventSleep: candidate.preventSleep === true,
         globalShowHide,
-        closeAction
+        closeAction,
+        autoCheckUpdate: candidate.autoCheckUpdate !== false
       }
     }
   }