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/char-export.md

@@ -0,0 +1,48 @@
+# Character export screen (`src/mods/charexport.cpp`)
+
+Request (2026-10-18): the export screen (Esc menu "Exportieren") was not
+adapted to widescreen (layout and clicks shifted), slots could not be
+cleared or overwritten; the screen and the save game window need black
+beside the background picture.
+
+## Layout
+
+See `hud.md`, "Full-screen windows": the 3D scene was stretched across the
+wide viewport; the window now draws with a viewport on the centred frame.
+
+## cUI_Character
+
+One window for the main menu's character selection / import and the export
+screen (vtable 0x894fa8, render vf44 0x70f160, events vf10 0x70e860).
+
+| Field | Meaning |
+|---|---|
+| +0x390 | mode: 4 selection, 5 export, 6 overwrite confirmation (7 import) |
+| +0x394 | previous mode |
+| +0x1b4 | 8 slot controls; hero id at +0x2f8 (0: empty) |
+| +0x1d6 | selected slot (char, -1 none) |
+| +0x3b4 | erase button ("UI_CHAR_ERASE_TOOLTIP") |
+
+| Address | Function |
+|---|---|
+| 0x70c760 | SetMode (mode), ret 4; entering mode 5 reloads the heroes |
+| 0x70c440 | SelectSlot (char slot), ret 4; shows the erase button in modes 4 / 7 only |
+| 0x70d0f0 | ExecuteAction (action, slot, extra), ret 0xc: 6 export into slot (`Save\Hero%.2d.pax`), 9 export with confirmation if occupied |
+| 0x70e650 | right-hand button: in mode 5 action 6 (empty slot) or 9 (occupied) |
+| 0x7f01a0 | HeroSlots_DeleteHero (slot + 1), thiscall on 0x7d7c40(), ret 4 |
+
+## Overwrite
+
+Measured with a log: exporting to an occupied slot was already the game's own
+path (action 9 -> mode 6 confirmation -> action 6). It only failed because
+clicks hit the wrong place before the layout fix.
+
+## Clear
+
+The erase button is created for every mode, and its click handler (vf10)
+does not check the mode: it opens the game's confirmation
+(UI_CHAR_ERASE_CONFIRM, cUI_BusyDlg), whose answer (0x72087b) calls
+DeleteHero. The mod shows the button in export mode whenever an occupied slot
+is selected (after SetMode / SelectSlot). After DeleteHero, if the screen is
+in export mode, it enters export mode again (reloads the heroes) and clears
+the selection; otherwise the erased hero stayed until the next export.