fix: add system tray icon for all platforms (macOS, Windows, Linux)

Tray was invisible on Windows/Linux (icon not bundled in asar) and
completely absent on macOS (early return guard). Generate dedicated
tray-sized icons, bundle them in the packaged app, and create tray
on all platforms with macOS Template image support.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

electron-builder.yml

@@ -6,6 +6,8 @@ directories:
 files:
   - out/**
   - package.json
+  - assets/icon.png
+  - assets/tray-icon*.png
 asar: true
 icon: assets/icon.png