Classic tray balloon notification mode

Summary¶

Add a deliberate native Windows notification-area balloon mode alongside modern Windows toasts and the existing custom eMule popup.

[x] Notification display mode is selected from explicit Windows toast, classic tray balloon, and custom popup choices.
[x] Fresh profiles default to Windows toast while existing saved enum values stay stable.
[x] Classic tray balloon mode uses Shell_NotifyIcon NIF_INFO balloons.
[x] Elevated or otherwise failed Windows toast attempts fall back to classic tray balloons before the custom popup.
[x] A separate Tweaks option can keep the tray icon visible.
[x] Classic tray balloon mode, toast fallback, and minimize-to-tray keep the tray icon visible when needed.
[x] Tray balloon clicks restore eMule without opening notification links.

NotifierDisplayMode values are stable: 0 custom popup, 1 Windows toast, 2 classic tray balloon.
AlwaysShowTrayIcon is independent of minimize-to-tray. Notification delivery can still force tray visibility while the setting is disabled.
Tray icon callbacks now use NOTIFYICON_VERSION_4 decoding.