v0.1.6

v0.1.6: Fix Snap issues and remove Flathub files

- Fix "Open link" to use system browser via shell.openExternal IPC
- Fix tray icon not showing by checking multiple icon paths
- Fix multiple tray slots by adding single instance lock and proper cleanup
- Fix Test button styling (add btn-secondary default state)
- Fix GTK file dialog font rendering in Snap with environment variables
- Add GTK_USE_PORTAL and GDK_BACKEND environment vars for Snap
- Remove all Flathub-related files (abandoned submission)