[codex] Polish launch screen UI

[KevinBigham]

Summary

• Add DESIGN.md as the active design source of truth for the MFD browser game.
• Rework the launch/new game screen into a clearer command-deck flow with responsive franchise selection, difficulty/scenario states, recovery/import clarity, and stronger start/demo actions.
• Add launch-specific CSS plus Visual Ralph screenshot evidence for desktop and mobile.

Validation

• eslint on changed app/test files
• tsc -p apps/web/tsconfig.json --noEmit
• vitest run NewGameScreen → 2 files, 13 tests passed
• Vite production build passed with the existing large chunk warning
• Headless Chrome desktop/mobile screenshots
• UltraQA CDP smoke: mobile overflow, ARIA state, scenario switching, invalid import recovery, payload injection guard, toggle stress, runtime responsiveness

Notes

• The broad Vitest run still has unrelated existing failures in older static-markup expectations and playtesting globals; this PR does not touch those areas.