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>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "agent-company",
-  "version": "0.4.90",
+  "version": "0.4.91",
   "private": true,
   "type": "module",
   "description": "Cross-platform Electron desktop control plane for local-first AI agent companies.",