refactor: Fullscreen handling

[fhscey]

This commit refactors fullscreen mode activation to be triggered only once upon user request and extends fullscreen support to Linux.

[liquidhelium]

Tested on windows. It worked fine.

[YuevUwU]

Consider to add translation for en-US (for macro fallback_langid), or this message waterfall will be still existing (in both Linux and Windows)

2025-12-31T11:05:20.261179200Z  WARN prpr_l10n::local: no translation found for item-fullscreen, returning key
2025-12-31T11:05:20.261522300Z  WARN prpr_l10n::local: no translation found for item-fullscreen-sub, returning key

[YuevUwU]

[BUG]
The Linux build can't expectedly cancel the fullscreen mode.
Disable fullscreen_mode will flash and still in fullscreen mode.
Tested on VirtualBox VM with these OS:

• Linux Mint 22.1 x86_64 + Xfce4 4.18 + Xfwm4 (X11)
• Fedora Linux 43 (Workstation Edition) x86_64 + GNOME 49.2 + Mutter (Wayland)

---

Current Behaviour of Implementation on Windows (Record-only, not bug or expected behaviour)

• Game does not enter fullscreen based on current config.fullscreen_mode setting
  • Reproduction: Exit fullscreen mode using Alt-F4, then re-enter the game
• If make window maximize, and enable fullscreen, and disable fullscreen, it will go back to window mode but not maximized