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/game/ui/draw.h

@@ -19,18 +19,35 @@ struct ScreenVertex {
     float u, v;
 };
 
-// Fills (x0, y0)-(x1, y1) with the ARGB colour `color`.
-inline void FillRect(void* device, float x0, float y0, float x1, float y1, uint32_t color) {
+// Fills (x0, y0)-(x1, y1), ARGB colours per corner: top left, top right,
+// bottom left, bottom right (blended between them).
+inline void FillCorners(void* device, float x0, float y0, float x1, float y1, uint32_t topLeft, uint32_t topRight,
+                        uint32_t bottomLeft, uint32_t bottomRight) {
     void** vtable = *static_cast<void***>(device);
     using SetTextureFn = HRESULT(__stdcall*)(void*, DWORD, void*);
     using DrawPrimitiveFn = HRESULT(__stdcall*)(void*, DWORD, DWORD, void*, DWORD, DWORD);
     reinterpret_cast<SetTextureFn>(vtable[kDeviceSetTexture])(device, 0, nullptr);
-    ScreenVertex quad[4] = {{x0, y0, 0, 1, color, 0, 0, 0},
-                            {x1, y0, 0, 1, color, 0, 1, 0},
-                            {x0, y1, 0, 1, color, 0, 0, 1},
-                            {x1, y1, 0, 1, color, 0, 1, 1}};
+    ScreenVertex quad[4] = {{x0, y0, 0, 1, topLeft, 0, 0, 0},
+                            {x1, y0, 0, 1, topRight, 0, 1, 0},
+                            {x0, y1, 0, 1, bottomLeft, 0, 0, 1},
+                            {x1, y1, 0, 1, bottomRight, 0, 1, 1}};
     reinterpret_cast<DrawPrimitiveFn>(vtable[kDeviceDrawPrimitive])(device, kTriangleStrip, kFvfScreenColourTexture,
                                                                      quad, 4, 0);
 }
 
+// Fills (x0, y0)-(x1, y1) with the ARGB colour `color`.
+inline void FillRect(void* device, float x0, float y0, float x1, float y1, uint32_t color) {
+    FillCorners(device, x0, y0, x1, y1, color, color, color, color);
+}
+
+// Top to bottom from `top` to `bottom`.
+inline void FillVertical(void* device, float x0, float y0, float x1, float y1, uint32_t top, uint32_t bottom) {
+    FillCorners(device, x0, y0, x1, y1, top, top, bottom, bottom);
+}
+
+// Left to right from `left` to `right`.
+inline void FillHorizontal(void* device, float x0, float y0, float x1, float y1, uint32_t left, uint32_t right) {
+    FillCorners(device, x0, y0, x1, y1, left, right, left, right);
+}
+
 } // namespace game