feat(frontend): fullscreen toggle, move indicators, material advantage & move-history rewind

[Yilkash]

Summary

Batched frontend enhancement for four assigned issues on a single branch. All changes are in the React/Vite/TypeScript frontend.

#124 — Fullscreen Mode Toggle

• Added a fullscreen toggle button (Maximize2 / Minimize2) to the game action bar using the HTML5 Fullscreen API.
• Added the f keyboard shortcut (ignored while typing in inputs).
• State tracks fullscreenchange so the icon reflects the real state.

#123 — Legal Move Dot Indicators & Capture Ring Highlights

• Empty legal-move squares now render the small dot marker.
• Capturable enemy pieces render a circular ring highlight instead of a dot.

#109 — Captured Pieces & Material Advantage

• Both player bars now show captured-piece icons (previously only the current player's bar had icons, keyed to the live turn).
• Added a real-time material point differential badge next to each player's avatar (e.g. +3, −5), computed from captured piece values in chessUtils.ts.

#112 — Interactive Move History Navigation / Board Playback Rewind

• Move history is fetched from GET /api/games/:code/moves and kept in the Zustand store (moveHistory, viewingIndex), refreshed after moves, on join/rejoin, and kept in sync via move_count.
• Playback controls in the action bar: jump-to-start, previous, next, and return-to-live, plus a "Move N of M" readout.
• Keyboard ← / → rewind/advance the board (ignored while typing).
• Clicking any earlier move in the history list rewinds the board; the last-move highlight follows the viewed move.
• The floating panel (was chat-only) now has Chat and Moves tabs — the Moves tab lists every move in algebraic notation (with +/# suffixes) and is fully clickable.
• While reviewing, the board is read-only (moves blocked) and a "Move N of M · Live" banner appears over the board.

Verification

• npm run lint passes on all touched files.
• npx tsc --noEmit passes with zero errors.
• npm test (Vitest) passes.
• Note: eslint . on the whole frontend/ still reports one pre-existing parsing error in src/pages/SpectatorPage.tsx:193 that also exists on master (it was merged broken in a previous PR) — left untouched per contribution scope.

Closes #124
Closes #123
Closes #112
Closes #109

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