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/widescreen.cpp

@@ -70,6 +70,7 @@ std::atomic<DWORD> g_menuThread = 0;
 // menu is showing. UI input is processed outside the menu thread.
 std::atomic<bool> g_lastSceneClassic = false;
 thread_local int g_loadingScreenDepth = 0;
+std::atomic<bool> g_wideMenuScreen = false;
 
 // D3DVIEWPORT7 (d3d.h is not in current Windows SDKs).
 struct Viewport7 {
@@ -346,7 +347,11 @@ bool IsClassicFrame() {
     if (g_renderWidth == 0) {
         return false;
     }
-    return g_loadingScreenDepth > 0 || GetCurrentThreadId() == g_menuThread.load();
+    return g_loadingScreenDepth > 0 || (GetCurrentThreadId() == g_menuThread.load() && !g_wideMenuScreen);
+}
+
+void SetWideMenuScreen(bool shown) {
+    g_wideMenuScreen = shown;
 }
 
 } // namespace mods::widescreen