Characters: campaigns, "Spielen" main menu, story unlocks

- One world per campaign and difficulty in a character file (sections
  ancaria<d> / underworld<d>; old world<d> renamed when read). Difficulty
  unlocks by the shared hero's story completions (hero header +0xed..),
  shared by both campaigns.
- Main menu: "Kampagne Ancaria", "Kampagne Underworld" and "Spielstaende"
  become "Spielen" (mods/mainmenu.cpp). The load window lists all
  characters, the world dialog offers both campaigns, "Neuer Charakter"
  asks for the campaign and opens character creation.
- Character window: no Underworld import mode (new Underworld heroes, all
  classes, slots light up), no "Importieren"; load window without
  "Speichern".

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

CMakeLists.txt

@@ -73,6 +73,7 @@ add_library(sacredK SHARED
     src/mods/userfolders.cpp
     src/mods/charfile.cpp
     src/mods/characters.cpp
+    src/mods/mainmenu.cpp
 )
 target_include_directories(sacredK PRIVATE src)
 target_compile_options(sacredK PRIVATE ${SACREDK_CXX_FLAGS})