feat: improve opening trainer flow and add deviation dialog

- Add tutor message guardrail to prevent rapid-fire messages
  - Track last message by move index
  - Wait for both player and opponent moves before speaking
  - Speak immediately when player deviates from theory

- Add deviation dialog with three options:
  - Continue Playing (Start Game) - transitions to game mode
  - Undo & Return to Opening - returns to theory
  - Explore This Variation - continues off-book practice

- Implement smooth game mode transition:
  - Pass opening context from trainer to game
  - Tutor welcomes player with context about their study
  - Computer makes first move if needed in starting position

- Fix generate-test-fixtures script:
  - Add safety check to prevent overwriting real data
  - Restore full opening database (12,379 openings)
  - Rebuild move index (12,377 sequences)
  - Add clear warnings about test fixtures

- Update ChessGame to accept openingContext prop
- Update Tutor to display contextual greeting in game mode

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

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