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>

CMakeLists.txt

@@ -67,6 +67,7 @@ add_library(sacred_re SHARED
     src/mods/runetrade.cpp
     src/mods/teleportpotion.cpp
     src/mods/guards.cpp
+    src/mods/charexport.cpp
 )
 target_include_directories(sacred_re PRIVATE src)
 target_compile_options(sacred_re PRIVATE ${SACRED_RE_CXX_FLAGS})