feat: add Camera Stiffness and Pivot Height settings for motion comfort

Camera Stiffness (default 20, range 5-100): controls how tightly the
camera follows the player. Higher values = less sway/lag. Users who
experience motion sickness can increase this to reduce floaty camera.

Camera Pivot Height (default 1.8, range 0-3): height of the camera
orbit point above the player's feet. Lower values reduce the
"detached/floating" feel that can cause nausea. Setting to 0 puts the
pivot at foot level (ground-locked camera).

Both settings saved to settings file and applied via sliders in the
Gameplay tab of the Settings window.

include/ui/game_screen.hpp

@@ -201,6 +201,8 @@ private:
     float pendingMouseSensitivity = 0.2f;
     bool pendingInvertMouse = false;
     bool pendingExtendedZoom = false;
+    float pendingCameraStiffness = 20.0f;  // Camera smooth speed (higher = tighter, less sway)
+    float pendingPivotHeight = 1.8f;       // Camera pivot height above feet (lower = less detached feel)
     float pendingFov = 70.0f;  // degrees, default matches WoW's ~70° horizontal FOV
     int pendingUiOpacity = 65;
     bool pendingMinimapRotate = false;