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>

src/game/ui/control.h

@@ -0,0 +1,24 @@
+#pragma once
+// cUI_Control2: flags at +0x10 (bit 0: visible; a hidden control still
+// renders but handles no events).
+
+#include <cstddef>
+#include <cstdint>
+
+namespace game {
+
+constexpr size_t kControlFlagsOffset = 0x10;
+constexpr uint32_t kControlFlagVisible = 1;
+
+inline bool ControlShown(const void* control) {
+    return (static_cast<const uint8_t*>(control)[kControlFlagsOffset] & kControlFlagVisible) != 0;
+}
+
+// cUI_Control2::SetFlags (0x731e70) / ClearFlags (0x731ee0), thiscall (flags),
+// ret 4; setting the visible flag calls the control's vf24 (show).
+inline void SetControlShown(void* control, bool shown) {
+    using FlagFn = void(__fastcall*)(void*, void*, uint32_t);
+    reinterpret_cast<FlagFn>(shown ? 0x00731e70 : 0x00731ee0)(control, nullptr, kControlFlagVisible);
+}
+
+} // namespace game