Add server-side gomoku (five-in-a-row) as game type 9 with 15x15 board,
2-player rooms, move validation, win detection in 4 directions, and
draw detection. Includes channel-based turn sync with mutex-protected
board access.
Add vanilla JS web client served as static files from the Go server:
WebSocket connection with base64 packet encoding, state machine
navigation, Canvas-based board renderer with gradient stones, hover
ghost, last-move highlight, and game-over overlay.
