Improve editor project management UI

[edisonlee0212]

Summary

• add a panel-managed editor layout and project content browser for browsing, selecting, dragging, and inspecting project assets
• add shared offscreen preview rendering plus asset thumbnail generation for textures, materials, and meshes, including interactive Asset Inspector previews
• apply the shared editor theme to the launcher, add light/dark theme support, remove the legacy theme, update editor launch sizing, and remove the DemoApp stormtrooper

Validation

• python Scripts\format_cpp.py --root EvoEngine_SDK --root EvoEngine_App --check
• git diff --check
• git diff --cached --check
• cmake --build out/build/vs2026-x64 --config RelWithDebInfo --target EvoEngineEditor EvoEngineLauncher
• python Scripts\install_apps.py --config RelWithDebInfo --incremental --no-open

Manual testing

• Manually tested milestone-by-milestone during development, including launcher/editor theme behavior, project browser thumbnails, Asset Inspector previews, thumbnail interaction/reset behavior, mesh framing, and updated editor launch sizing.

Test binaries

• C:\Users\lllll\Documents\GitHub\EvoEngine\out\build\vs2026-x64\EvoEngine_App\RelWithDebInfo\EvoEngineEditor.exe
• C:\Users\lllll\Documents\GitHub\EvoEngine\out\build\vs2026-x64\EvoEngine_App\RelWithDebInfo\EvoEngineLauncher.exe
• C:\Users\lllll\Documents\GitHub\EvoEngine\out\install\vs2026-x64\bin