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/shared/api.ts

@@ -8,6 +8,7 @@ import type {
   SessionOpenResult
 } from './ipc'
 import type { AppSettings } from './settings'
+import type { ReleaseNote, UpdateState } from './ipc'
 import type {
   HostKeyAction,
   HostKeyPromptEvent,
@@ -105,4 +106,11 @@ export interface AppApi {
   /** json = serialized dockview layout (DockviewApi.toJSON()) */
   saveLayout(meta: LayoutMeta, json: string): Promise<void>
   deleteLayout(id: string): Promise<void>
+
+  // ---- updater (domestic feed first, GitHub fallback) ----
+  updateCheck(): Promise<UpdateState>
+  updateDownload(): Promise<void>
+  updateInstall(): void
+  updateChangelog(): Promise<ReleaseNote[]>
+  onUpdateState(cb: (s: UpdateState) => void): () => void
 }