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>

package.json

@@ -23,6 +23,7 @@
     "axios": "^1.6.2",
     "cheerio": "^1.0.0-rc.12",
     "electron-store": "^8.1.0",
+    "electron-updater": "^6.8.3",
     "nedb": "^1.8.0",
     "uuid": "^9.0.1",
     "xml2js": "^0.6.2"
@@ -53,6 +54,10 @@
     "directories": {
       "output": "release"
     },
+    "publish": {
+      "provider": "generic",
+      "url": "https://your-update-server.example.com/media-center/"
+    },
     "files": [
       "dist/**/*",
       "node_modules/**/*",