Steam Deck scaling fix: bottom controls and controller hints no longer cut off after switching tabs.
- Fixed font scaling to use content area dimensions instead of full window, preventing zoom-in on tab switch
- Reserved bottom space for buttons and controller hints overlay in font calculations
- Reduced timer font size ratio and cap for better fit on small screens
- Scoped deck-mode button CSS to avoid inflating SpinRow controls
- Added Fullscreen preference (Auto/On/Off) in Controller & Deck Mode settings
- Replaced hardcoded 1280×800 baseline with dynamic allocation-based sizing
- Debounced font updates on resize to avoid flicker during fullscreen transitions
- Reduced controller hints overlay size for less screen obstruction
