Full-screen support

Show Stik as a floating overlay above full-screen apps.

My workflow keeps apps like browsers and IDEs in dedicated full-screen spaces. To use Stik, I have to swipe away from my current space entirely, which breaks focus and flow.

Requested behavior: When triggering Stik's hotkey from any space (including full-screen app spaces), the window should appear as a floating overlay on top of the current app, without switching spaces.

According to Claude, this requires setting NSWindowCollectionBehavior.canJoinAllSpaces and raising the window level above NSNormalWindowLevel on macOS.