Use game-root ImGui launcher

[mstan]

Switches this game repo to the game-root ImGui/recomp-ui launcher setup and removes the stale RmlUi-era launcher wiring/references.

Pins:

• psxrecomp: f86087ad94e53fba5ec0bb146ae94f789c60803a (Remove stale RmlUi launcher remnants psxrecomp#76)
• recomp-ui: 44f549c0f159df343cba9d8c3842dbc7e592eb08

Verification run locally across the game migration set:

• exact framework/UI gitlink and checkout SHAs matched
• tracked/index grep for RmlUi/Rml::/PSX_LAUNCHER/launcher.rml/launcher_art returned no hits
• no manual recomp-ui include/recomp_target_launcher_ui wiring remained in game CMakeLists.txt
• git diff --check and git diff --cached --check passed

Build note: the local CMake/Ninja path issue was caused by devkitPro MSYS CMake being picked with MSYS64 Ninja/compilers; using C:\msys64\mingw64\bin\cmake.exe fixes compiler detection. A full runtime configure still needs generated BIOS C files supplied locally/CI.