Export screen: widescreen frame, black sides, slots can be cleared

- cUI_Character (export) and cUI_Savegame draw inside the centred 1024x768
  frame (viewport) with the render target beside it cleared black; the
  export screen's 3D heroes were stretched across the wide viewport, away
  from their labels and hit areas.
- Export screen shows the game's erase button for an occupied slot and
  reloads the heroes after a delete. Overwriting was already the game's
  own path once clicks hit the right slot.
- game/ui/control.h: shared show/hide for cUI_Control2.

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

docs/hud.md

@@ -126,3 +126,19 @@ offset, that reset reached past the render target, `SetViewport` failed and
 the small viewport stayed set: the cursor (drawn after the UI) was only
 visible inside it. A viewport at x 0 across the whole render width is now
 passed unshifted.
+
+## Full-screen windows: frame viewport and black sides (2026-10-18)
+
+The character export screen (Esc menu "Exportieren", cUI_Character) and the
+save game window (cUI_Savegame) cover the 1024 frame with a picture. Request:
+black instead of the world beside it, and the export screen adapted to the
+wide view. Measured on a screenshot: the export screen's 3D scene (heroes on
+their stones) is projected for a 1024x768 viewport; across the widened one
+it was stretched 1.8x wide, away from the slot labels and their hit areas.
+
+Their render hooks (`HUD_RENDER_FRAMED`, `HUD_RENDER3_FRAMED`) run the window
+inside `FrameScope`: while the window is visible, the render target left and
+right of the centred frame is cleared black, then the viewport is set to the
+frame (dx, 0, 1024x768) and restored afterwards. `Clear` clips to the
+viewport, so the sides are cleared first. Both calls go to the unhooked
+device methods (`IDirect3DDevice7` 10 Clear, 13 SetViewport, 15 GetViewport).