Replace the tkinter panel with a GTK3 control panel (gtkui.py) + settings
dialog (gtksettings.py): rounded CSD window, gradient background, circular icon
avatars, hotkey pills, hover states, Ubuntu font. Runs on the GTK main loop,
unified with the AppIndicator tray (no tkinter event-loop pump). Status updates
marshalled via GLib.idle_add. Removes the old tkinter gui.py.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
