Suppress movement after teleport/portal, add shadow distance slider

- Add movementSuppressTimer to camera controller that forces all movement
  keys to read as false, preventing held W key from carrying through
  loading screens (fixes always-running-forward after instance portals)
- Increase shadow frustum default from 60 to 72 units (+20%)
- Make shadow distance configurable via setShadowDistance() (40-200 range)
- Add shadow distance slider in Video settings tab (persisted to config)

include/ui/game_screen.hpp

@@ -87,6 +87,7 @@ private:
     bool pendingVsync = false;
     int pendingResIndex = 0;
     bool pendingShadows = true;
+    float pendingShadowDistance = 72.0f;
     bool pendingWaterRefraction = false;
     int pendingMasterVolume = 100;
     int pendingMusicVolume = 30;