Enhance system tray and Wayland support, improve dialogs, and modernize build:

- Add detailed handling for sessions without a system tray (e.g., GNOME, sway): ensure windows remain reachable, clarify minimized behavior, and add notification for missing tray (#166).
- Ensure consistent dialog behavior: bring main window forward before displaying child dialogs to prevent Wayland layering issues.
- Adjust window centering logic for multi-monitor and Wayland environments.
- Pin Qt6 minimum version to 6.4 for compatibility with Ubuntu 24.04 and Debian bookworm.
- Update README and changelog with Wayland limitations and system tray behavior.
- Add GUI test support for missing tray scenarios and refine instance-check logic to avoid misidentification of child processes.