feat(desktop): packaging + self-hosted auto-update (electron-builder/updater)

- package.json build config: NSIS installer, app icon, generic publish provider →
  https://remote.bizgaze.com/downloads/ (self-hosted update feed).
- main.js: electron-updater checks the feed on launch + every 6h, downloads in the
  background, installs on restart. Active only in packaged builds.
- build/icon.ico app icon; PACKAGING.md documents build/release/signing.
- Key design: web/UI changes reach installed apps instantly (they load the live server);
  only native shell changes need an auto-updated build.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

desktop/package.json

@@ -7,6 +7,9 @@
     "start": "electron .",
     "dist": "electron-builder"
   },
+  "dependencies": {
+    "electron-updater": "^6.3.9"
+  },
   "devDependencies": {
     "electron": "^31.0.0",
     "electron-builder": "^24.13.3"
@@ -14,9 +17,20 @@
   "build": {
     "appId": "com.bizgaze.connect.desktop",
     "productName": "Biz Connect",
-    "files": ["main.js", "preload.js", "assets/**"],
-    "win": { "target": "nsis" },
-    "mac": { "target": "dmg", "category": "public.app-category.business" },
-    "linux": { "target": "AppImage" }
+    "directories": { "buildResources": "build", "output": "dist" },
+    "publish": [
+      { "provider": "generic", "url": "https://remote.bizgaze.com/downloads/" }
+    ],
+    "win": { "target": "nsis", "icon": "build/icon.ico" },
+    "nsis": {
+      "oneClick": false,
+      "perMachine": false,
+      "allowToChangeInstallationDirectory": true,
+      "createDesktopShortcut": true,
+      "createStartMenuShortcut": true,
+      "shortcutName": "Biz Connect"
+    },
+    "mac": { "target": "dmg", "category": "public.app-category.business", "icon": "build/icon.ico" },
+    "linux": { "target": "AppImage", "category": "Network" }
   }
 }