feat(tictactoe): integrate leaderboard UI, provider API hook, and new styles
- Add *_BKP* ignore rule to .gitignore
- Insert Leaderboard component into TicTacToe screen
- Add getLeaderboardTop() API method to NakamaProvider and expose via context
- Add full leaderboard polling logic (interval-based) in new Leaderboard.tsx
- Style leaderboard in styles.css (rows, rank, name, score, empty state, card UI)
- Modernize TicTacToe UI styles (board, squares, buttons, layout)
- Replace matchmaking view with leaderboard integration
- Import ApiLeaderboardRecordList and ApiMatch types cleanly