fix(android): 修复导出备份后返回启动屏幕的问题

问题原因：
- 当文件选择器 (ActivityResultContracts.CreateDocument) 启动时，
  Android 可能会销毁并重新创建 Activity（配置更改）
- startupComplete、pendingExportJson、pendingExportAddress 使用 remember
  存储状态，在 Activity 重建时会丢失
- startupComplete 重置为 false 导致显示启动检查屏幕

修复方案：
- 将 startupComplete 从 remember 改为 rememberSaveable
- 将 pendingExportJson 和 pendingExportAddress 从 remember 改为 rememberSaveable
- rememberSaveable 会通过 Android 的 savedInstanceState 机制持久化状态

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

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

@@ -13,6 +13,7 @@ import androidx.activity.result.contract.ActivityResultContracts
 import androidx.compose.foundation.layout.*
 import androidx.compose.material3.*
 import androidx.compose.runtime.*
+import androidx.compose.runtime.saveable.rememberSaveable
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.platform.LocalContext
 import androidx.hilt.navigation.compose.hiltViewModel
@@ -114,8 +115,9 @@ fun TssPartyApp(
 
     // Export/Import file handling
     val context = LocalContext.current
-    var pendingExportJson by remember { mutableStateOf<String?>(null) }
-    var pendingExportAddress by remember { mutableStateOf<String?>(null) }
+    // Use rememberSaveable to persist across configuration changes (e.g., file picker activity)
+    var pendingExportJson by rememberSaveable { mutableStateOf<String?>(null) }
+    var pendingExportAddress by rememberSaveable { mutableStateOf<String?>(null) }
 
     // File picker for saving backup
     val createDocumentLauncher = rememberLauncherForActivityResult(
@@ -180,7 +182,8 @@ fun TssPartyApp(
     }
 
     // Track if startup is complete
-    var startupComplete by remember { mutableStateOf(false) }
+    // Use rememberSaveable to persist across configuration changes (e.g., file picker activity)
+    var startupComplete by rememberSaveable { mutableStateOf(false) }
 
     // Handle success messages
     LaunchedEffect(uiState.successMessage) {