feat(brand): apply Fabric app + tray icons

- assets/icon.png (512) -> BrowserWindow icon; build/icon.png (1024)
  for electron-builder (linux uses it directly, mac/win generated from
  it); package.json build.files += assets/**, build.icon set.
- Tray: replace the placeholder base64 data-URL with the designed
  no-text tray icon (assets/tray.png 22 + tray@2x.png 44) via
  nativeImage.createFromPath.

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

package.json

@@ -33,9 +33,11 @@
       "main.js",
       "preload.js",
       "offline.html",
-      "package.json"
+      "package.json",
+      "assets/**"
     ],
     "asar": true,
+    "icon": "build/icon.png",
     "linux": {
       "target": [
         "AppImage",