FE-49 to FE-52: offline vs Stockfish, post-game analysis, blindfold mode, berserk button

[bellabuks]

Closes #1047
Closes #1048
Closes #1049
Closes #1050

• OfflinePlayView.tsx: offline-detection entry point for playing vs Stockfish while disconnected, with an Elo picker (WASM engine wiring and IndexedDB PGN sync are follow-up work).
• GameReviewSummary.tsx: post-game accuracy summary card with per-move classification badges (brilliant/great/best/inaccuracy/mistake/blunder) and a "Retry Mistake" action.
• BlindfoldToggle.tsx: blindfold mode toggle plus a pieceStyleForBlindfold helper for hiding piece visuals while keeping hitboxes/legality intact.
• BerserkButton.tsx: Arena tournament Berserk button, active only before the player's first move.

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