macOS menu bar tray icon uses full-color purple app icon

Problem

On macOS, Meetily currently uses the full-color app icon for the menu bar tray icon. That makes the tray item appear purple instead of matching the native monochrome system menu bar icons.

Expected behavior

The menu bar tray icon should use a template-safe monochrome Meetily icon so macOS can render it correctly in light and dark mode.

Proposed fix

Use a dedicated monochrome tray asset derived from the existing Meetily app icon, load it for the Tauri tray icon, and mark it with icon_as_template(true).

Related PR

https://github.com/Zackriya-Solutions/meetily/pull/420