fix(android): registerParty 前等待 channel READY

问题：connect() 是异步的，registerParty() 在 channel 还是 CONNECTING 时就被调用，导致 RST_STREAM 错误

修复：在 registerParty() 开头等待 channel READY 后再发送请求

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/mpc-system/services/service-party-android/app/src/main/java/com/durian/tssparty/data/remote/GrpcClient.kt

@@ -593,6 +593,15 @@ class GrpcClient @Inject constructor() {
         partyRole: String = "temporary",
         version: String = "1.0.0"
     ): Result<Boolean> = withContext(Dispatchers.IO) {
+        // 必须等待 channel READY 后才能注册
+        Log.d(TAG, "registerParty: Waiting for channel READY...")
+        val isReady = waitForChannelReady(CONNECTION_TIMEOUT_SECONDS * 1000)
+        if (!isReady) {
+            Log.e(TAG, "registerParty: Channel not ready after timeout")
+            return@withContext Result.failure(Exception("Channel not ready"))
+        }
+        Log.d(TAG, "registerParty: Channel is READY, proceeding with registration")
+
         // Save for re-registration
         registeredPartyId = partyId
         registeredPartyRole = partyRole