Shared chest, menu after a game, dialogs over 16:9 screens

- Shared chest: emptied and not saved from the moment a save game is read
  until it reloads; a hero saved inside a load wrote the new world's
  objects (stale ids) as the chest's items.
- Main menu after a game: shown 16:9 screens are tracked on the menu thread
  only and expire after 2 frames; a destroyed in-game window kept the menu
  unframed (old pictures, frame at the left).
- Dialogs over a 16:9 screen in the main menu are centred, their dimming
  covers the screen.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/mods/screen.h

@@ -50,8 +50,9 @@ constexpr uint32_t kRowSelectedColour = 0xc0604818;
 // The background of the menu screens across the whole render target (also
 // beside the main menu's centred frame and full-screen windows in a game).
 void DrawBackground(void* device);
-// Whether a screen is shown (the main menu then does not draw centred).
-bool AnyShown();
+// Called once per main menu frame (menu thread): whether a screen is shown
+// (the main menu then does not draw centred).
+bool BeginMenuFrame();
 
 // Draws the screen of `window`: the background across the whole render
 // target, then body(context) in canvas coordinates.