Characters: 16:9 "Spielen" screen, autosave interval option

- Shared screen kit (mods/screen, docs/screens.md): a 1365x768 canvas
  centred in the render width on our own dark background, the game's
  frames and buttons; the main menu renders full width while it is shown.
- The load window (cUI_Savegame) is replaced by the Spielen screen:
  character list, hero panel, campaign x difficulty grid, save/autosave
  choice, rename/delete/reset/new character dialogs.
- Migrated characters without a shared hero load their hero's world
  first, then start the new world.
- Options, Allgemein: autosave interval (Aus/5/10/15/30 minutes).

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

src/mods/hud.h

@@ -30,4 +30,14 @@ int HandleCentred(void* event, int (*body)(void* context), void* context);
 // with a viewport on it (as the save game and export screens).
 int RenderFramed(void* window, void* device, int (*body)(void* context), void* context);
 
+// 16:9 screens (mods/screen.h): laid out on a 1365x768 canvas, centred in
+// the render width. RenderCanvas / HandleCanvas run body(context) with drawing,
+// cursor and mouse events in canvas coordinates; CanvasOffsetX is the canvas'
+// left edge in the render target.
+constexpr int kCanvasWidth = 1365;
+constexpr int kCanvasHeight = 768;
+int CanvasOffsetX();
+int RenderCanvas(void* window, int (*body)(void* context), void* context);
+int HandleCanvas(void* event, int (*body)(void* context), void* context);
+
 } // namespace mods::hud