Add persistent settings UI with audio, video, and look controls
- Add a full settings screen with persistent prefs and separate audio buses.
- Replace the placeholder settings tab with controls for master/SFX/music volume, fullscreen, V-Sync, show FPS, look sensitivity, and invert Y, backed by a Settings singleton saved under the SDL pref path.
- Add a Slider widget and layout tweaks so the pause UI can host these options.
- Route volumes through MasterSpeaker/SfxSrc/MusicSrc,
- Support exit to title (stub) or desktop via the game-mode loop.