[Bug] WebSocket silently drops slow clients; client has no reconnection

[dev-rkb]

Description

Two related reliability issues:

1. packages/server/game/hub.go:69-74 — When a client's Send
   channel is full, the hub closes the channel and removes the client
   from the room. The peer receives no error or game_over frame, so
   the UI freezes with no indication.

2. packages/web/src/app/chess/page.tsx:31-71 — The frontend does
   not reconnect on socket close. After a network blip, the player must
   manually refresh the page to resume.

Proposed fix

Server:

• Replace silent close with a WSMessage{Type: "disconnect", payload: {reason: "slow_consumer"}} sent best-effort before closing.
• Add a pong handler and a read deadline on the websocket so dead
  connections are detected within ~60s instead of relying on the
  client's next write.

Client:

• Extract the WebSocket effect into packages/web/src/hooks/useChessSocket.ts
  (the empty hooks/ dir already exists for this).
• Implement exponential backoff reconnection (1s, 2s, 4s, 8s, cap at 30s).
• On reconnect, resend the join frame; on receiving game_update for
  the same gameId, re-hydrate state.
• Show a small "Reconnecting..." banner when readyState is not OPEN.

Acceptance criteria

• A client whose Send buffer fills receives a disconnect frame
  before the connection closes
• Refreshing the page mid-game is no longer required after a
  transient network blip; the board re-syncs within 30s
• A useChessSocket hook exists under packages/web/src/hooks/

Files

• packages/server/game/hub.go
• packages/server/handler/websocket.go
• packages/web/src/hooks/useChessSocket.ts (new)
• packages/web/src/app/chess/page.tsx