Add Render Debug GUI toggle to settings modal

The lil-gui render-settings panel (createDebugGui) has been an orphaned
export since the WebGL renderer landed — no caller, no hotkey, no menu
entry. Wire it to a button in SettingsModal that toggles the panel on
and off, and group it with the existing Performance Overlay toggle
under a new "Development Only" section above the Exit Game block.

src/client/InputHandler.ts

@@ -84,6 +84,8 @@ export class CloseViewEvent implements GameEvent {}
 
 export class RefreshGraphicsEvent implements GameEvent {}
 
+export class ToggleRenderDebugGuiEvent implements GameEvent {}
+
 export class TogglePerformanceOverlayEvent implements GameEvent {}
 
 export class ToggleStructureEvent implements GameEvent {