fix(server): make RustInterface.serverStarted volatile

[0xbrayo]

Part of #333.

RustInterface.serverStarted is shared across threads:

• the IO coroutine that starts the server sets it to true,
• a main-thread Handler sets it to false when the server exits,
• BackgroundService's sanitized-hostname-rewrite thread polls it in a while (RustInterface.serverStarted) Thread.sleep(...) loop.

Without @Volatile, the Java memory model doesn't guarantee that the polling thread ever sees the main thread's write, so the queued hostname rewrite could wait forever.

Change

• Mark the field @Volatile.

Testing

• ./gradlew :mobile:testStandardDebugUnitTest passes.

[greptile-apps]

Confidence Score: 5/5

[Medium impact] This PR appears safe to merge.

Summary

Adds @Volatile to RustInterface.serverStarted so the hostname-rewrite thread can see when the server exits.

• The new comment explains why the field needs cross-thread visibility.
• No actionable issues found.

_{Reviews (1) · Last reviewed commit: "fix(server): make RustInterface.serverSt..." · Reviewed by Greptile}

[ErikBjare]

Thanks @0xbrayo. Master already has this: #324 made serverStarted @Volatile (with the startServerTask() check-and-set under a lock), and its comment calls out the hostname-rewrite reader in BackgroundService. Rebasing this branch leaves an empty diff, so I'm closing it as superseded by #324.