Fix private room reconnect recovery

[coder13]

Summary

• preserve an in-progress solve and its pending result while the server restarts
• reconnect room sockets without duplicate joins and make same-room joins idempotent
• remember one successful private-room password per room on the device, reuse it after refresh or reconnect, and clear it when rejected or no longer needed
• show the private-room password form and inline error when automatic rejoin cannot restore access
• allow existing private rooms to be edited without replacing their password
• redact room passwords from socket logs

Why

A deploy temporarily disconnects active racers. Timing can continue locally, but the client must rejoin the room before submitting the saved result. Private rooms were especially fragile after a refresh because the password existed only in Redux memory, and duplicate buffered joins could leave the client disconnected from the room even after Socket.IO recovered.

This keeps the solve local during the interruption, retries room access with the last successfully used password, and submits the result once the room join succeeds. If that password is no longer valid, it is removed and the normal password form is shown.

Validation

• yarn lint
• yarn test — 75 client tests and 79 server tests passed
• yarn build