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>

docs/screens.md

@@ -75,3 +75,16 @@ window's own pictures removed.
 
 - Spielen (the load window, cUI_Savegame): docs/characters.md, "Spielen
   screen".
+
+## Fixes (2026-10-21)
+
+- The main menu came back unframed (old pictures, frame at the left) after a
+  game: the set of shown 16:9 screens kept a window of the in-game UI that
+  was destroyed while shown. Now only screens drawn on the menu thread count,
+  and an entry expires after 2 menu frames without being drawn
+  (`screen::BeginMenuFrame`).
+- Dialogs over a 16:9 screen in the main menu (e.g. "Spielstand laden?") sat
+  at the left with the dimming 1024 wide: `hud::OffsetFor` gave every window
+  offset 0 while the menu showed. Now only while the menu frame is drawn
+  centred (it shifts the windows itself); otherwise dialogs are centred and
+  their full-layout dimming stretched, as in a game.