feat(minesweeper): classic Minesweeper with chords and best times

Summary

• Adds src/minesweeper/: an original open-source Minesweeper matching documented classic Windows board sizes and rules (Beginner 9×9/10, Intermediate 16×16/40, Expert 16×30/99).
• Implements reveal, flag, chord, flood fill, first-click safety, mine counter, timer, face/reset, win/lose, and localStorage best times — original visuals (no proprietary assets).
• Vitest coverage for board generation, reveal/flag/chord, win/lose, and first-click safety (npm test: 25 passed). Amplify + customHttp.yml registered like other apps.
• Root Games README table intentionally not updated (same as other pending app PRs such as document-scanner); comparison matrix lives in src/minesweeper/README.md.

Test plan

• cd src/minesweeper && npm ci && npm test && npm run build
• Play Beginner: first click never dies; zeros flood; flags decrement counter; timer starts on first click
• Chord a numbered cell when adjacent flags match; mis-flagged chord loses
• Win a Beginner game and confirm best time persists after reload
• Switch Intermediate / Expert and reset via the face button / R