feat: switch to electron-updater for auto-updates

Replaces the custom server-poll updater (which depended on a placeholder
serverUrl 'https://your-server.com/api' and never fired in practice) with
electron-updater. Now a banner appears automatically when a new release is
published — auto-download in background, in-place install via quitAndInstall,
delta updates via blockmap.

Changes:
- Add electron-updater dep, build.publish (provider: generic, placeholder URL)
- Rewrite UpdaterManager around autoUpdater events (available/progress/
  downloaded/error), forwarded to renderer via window.webContents.send
- Drop hardcoded APP_VERSION constant; main uses app.getVersion(), renderer
  fetches via new GET_APP_VERSION IPC channel
- IPC: drop DOWNLOAD_UPDATE (autoDownload handles it), add INSTALL_UPDATE
  + GET_APP_VERSION
- VersionInfo reshaped (currentVersion field, no downloadUrl/mandatory);
  add UpdateProgress and UpdateCheckResult types
- UpdateNotification: 3-phase UI (downloading with progress bar,
  ready with restart-and-install, hidden); App.tsx tracks phase state

TODO before first real release:
- Replace build.publish.url placeholder with the actual generic host
- Bump version, run package:win, upload latest.yml + .exe + .blockmap to host

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/shared/constants.ts

@@ -1,5 +1,3 @@
-export const APP_VERSION = '1.0.1';
-
 export const DEFAULT_CONFIG_SERVER_URL = 'https://your-server.com/api';
 
 export const DEFAULT_PROXY_PORT = 10808;