Quit via File -> Exit / Alt+F4 instead of Escape

Escape was a leftover spike convenience that quit the tool. Esc is a common
in-game key, so dropping it from a borderless mirror was too easy to do by
accident. Remove the Escape-quit handler and add a File -> Exit menu item
(shown with its Alt+F4 shortcut). Alt+F4 already worked via DefWindowProc ->
WM_CLOSE -> WM_DESTROY; the menu item sets a one-shot flag the main loop reads.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

host/src/main.cpp

@@ -241,6 +241,11 @@ int run()
 		}
 		coop::apply_layout_end_frame(); // clear the one-shot "Reset layout" force
 
+		if (ui.request_quit) // File -> Exit
+		{
+			break;
+		}
+
 		// Forward the host window's mouse/keyboard into the game (when the MKB
 		// subsystem is on, we're focused, and ImGui isn't using the event).
 		coop::forward_mkb_frame(injection, window.hwnd(), capture.mirroring(), capture.source_hooked());