Implement actual circuit breaker behavior (not only heartbeat timeout)

[jonasew]

(We could also agree to remove circuit breaker as a part of our specification if time does not allow it)

Problem: Heartbeat exists, breaker state machine does not.
Target files (new + touched):

• GameNetworkHandler.java
• FirebaseAPI.java

Refactor:

• Add a small breaker component with closed/open/half-open states.
• Trip breaker after configurable consecutive failures/timeouts.
• While open, block non-essential network writes and surface reconnecting state.
• Use half-open probe to recover.

Acceptance criteria:

• After repeated heartbeat/write failures, network requests are throttled/paused.
• Recovery resumes automatically once probe succeeds.