Add proportional zoom and reduced default max zoom with extended toggle

Zoom speed now scales with distance for fine control near the character.
Default max zoom out reduced from 50 to 33 units. New "Extended Camera
Zoom" toggle in Gameplay settings restores the full 50-unit range.

include/ui/game_screen.hpp

@@ -94,6 +94,7 @@ private:
     int pendingActivityVolume = 100;
     float pendingMouseSensitivity = 0.2f;
     bool pendingInvertMouse = false;
+    bool pendingExtendedZoom = false;
     int pendingUiOpacity = 65;
     bool pendingMinimapRotate = false;
     bool pendingMinimapSquare = false;