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/types.ts

@@ -59,11 +59,21 @@ export interface ActiveTab {
 // Version Check Types
 export interface VersionInfo {
   latestVersion: string;
-  updateAvailable: boolean;
-  downloadUrl: string;
-  changelog: string;
+  currentVersion: string;
   releaseDate: string;
-  mandatory: boolean;
+  changelog: string;
+}
+
+export interface UpdateProgress {
+  percent: number;
+  bytesPerSecond: number;
+  transferred: number;
+  total: number;
+}
+
+export interface UpdateCheckResult {
+  updateAvailable: boolean;
+  latestVersion: string;
 }
 
 // Proxy Types
@@ -113,7 +123,8 @@ export enum IPC_CHANNELS {
 
   // Version
   CHECK_VERSION = 'version:check',
-  DOWNLOAD_UPDATE = 'version:download',
+  INSTALL_UPDATE = 'version:install',
+  GET_APP_VERSION = 'app:version',
 
   // Settings
   GET_SETTINGS = 'settings:get',