fix: biometric retry + sync change detection race condition

Biometric:
- Handle onAuthenticationError with auto-retry (except user cancel)
- Show lock screen with proper UI and an Unlock button as fallback
- Add subtitle clarifying fingerprint/PIN options

Sync engine:
- Fix data race: async coroutines now return FileOutcome instead of
  mutating shared vars/list concurrently (was causing file states to
  not be saved, so every sync re-transferred all files)
- Fix remoteChanged: use || instead of && so either etag or
  modifiedAt change is enough to detect a remote modification

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/src/main/kotlin/com/syncflow/MainActivity.kt

@@ -10,14 +10,23 @@ import androidx.biometric.BiometricManager.Authenticators.BIOMETRIC_STRONG
 import androidx.biometric.BiometricManager.Authenticators.DEVICE_CREDENTIAL
 import androidx.biometric.BiometricPrompt
 import androidx.compose.foundation.background
+import androidx.compose.foundation.layout.Arrangement
 import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.Spacer
 import androidx.compose.foundation.layout.fillMaxSize
-import androidx.compose.material3.CircularProgressIndicator
+import androidx.compose.foundation.layout.height
+import androidx.compose.material.icons.Icons
+import androidx.compose.material.icons.filled.Lock
+import androidx.compose.material3.Button
+import androidx.compose.material3.Icon
 import androidx.compose.material3.MaterialTheme
 import androidx.compose.material3.Surface
+import androidx.compose.material3.Text
 import androidx.compose.runtime.*
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
+import androidx.compose.ui.unit.dp
 import androidx.core.content.ContextCompat
 import androidx.core.splashscreen.SplashScreen.Companion.installSplashScreen
 import androidx.lifecycle.lifecycleScope
@@ -47,9 +56,22 @@ class MainActivity : AppCompatActivity() {
                     SyncFlowNavGraph(rememberNavController())
                 }
                 if (isLocked) {
-                    LockOverlay()
+                    var showRetry by remember { mutableStateOf(false) }
+                    LockOverlay(
+                        showRetry = showRetry,
+                        onRetry = {
+                            showRetry = false
+                            showBiometricPrompt(
+                                onSuccess = { isLocked = false },
+                                onFailed = { showRetry = true },
+                            )
+                        },
+                    )
                     LaunchedEffect(Unit) {
-                        showBiometricPrompt(onSuccess = { isLocked = false })
+                        showBiometricPrompt(
+                            onSuccess = { isLocked = false },
+                            onFailed = { showRetry = true },
+                        )
                     }
                 }
             }
@@ -75,24 +97,36 @@ class MainActivity : AppCompatActivity() {
             BiometricManager.BIOMETRIC_SUCCESS
     }
 
-    private fun showBiometricPrompt(onSuccess: () -> Unit) {
+    private fun showBiometricPrompt(onSuccess: () -> Unit, onFailed: () -> Unit) {
         val executor = ContextCompat.getMainExecutor(this)
         val prompt = BiometricPrompt(this, executor, object : BiometricPrompt.AuthenticationCallback() {
             override fun onAuthenticationSucceeded(result: BiometricPrompt.AuthenticationResult) {
                 onSuccess()
             }
+            override fun onAuthenticationError(errorCode: Int, errString: CharSequence) {
+                // User cancelled or lockout — re-show the prompt so they can retry
+                if (errorCode != BiometricPrompt.ERROR_USER_CANCELED &&
+                    errorCode != BiometricPrompt.ERROR_NEGATIVE_BUTTON) {
+                    showBiometricPrompt(onSuccess, onFailed)
+                } else {
+                    onFailed()
+                }
+            }
+            override fun onAuthenticationFailed() {
+                // Wrong finger/face — BiometricPrompt handles retries internally, no action needed
+            }
         })
         val promptInfo = if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.R) {
             BiometricPrompt.PromptInfo.Builder()
                 .setTitle("Unlock SyncFlow")
-                .setSubtitle("Confirm your identity to continue")
+                .setSubtitle("Use fingerprint or device PIN")
                 .setAllowedAuthenticators(BIOMETRIC_STRONG or DEVICE_CREDENTIAL)
                 .build()
         } else {
             BiometricPrompt.PromptInfo.Builder()
                 .setTitle("Unlock SyncFlow")
-                .setSubtitle("Confirm your identity to continue")
-                .setNegativeButtonText("Cancel")
+                .setSubtitle("Use fingerprint to continue")
+                .setNegativeButtonText("Use PIN")
                 .build()
         }
         prompt.authenticate(promptInfo)
@@ -100,13 +134,28 @@ class MainActivity : AppCompatActivity() {
 }
 
 @Composable
-private fun LockOverlay() {
+private fun LockOverlay(showRetry: Boolean, onRetry: () -> Unit) {
     Box(
         modifier = Modifier
             .fillMaxSize()
             .background(MaterialTheme.colorScheme.background),
         contentAlignment = Alignment.Center,
     ) {
-        CircularProgressIndicator()
+        Column(
+            horizontalAlignment = Alignment.CenterHorizontally,
+            verticalArrangement = Arrangement.Center,
+        ) {
+            Icon(Icons.Default.Lock, null, modifier = Modifier.height(48.dp), tint = MaterialTheme.colorScheme.primary)
+            Spacer(Modifier.height(16.dp))
+            Text("SyncFlow is locked", style = MaterialTheme.typography.titleMedium)
+            Spacer(Modifier.height(8.dp))
+            if (showRetry) {
+                Spacer(Modifier.height(8.dp))
+                Button(onClick = onRetry) { Text("Unlock") }
+            } else {
+                Text("Use fingerprint or PIN to unlock", style = MaterialTheme.typography.bodySmall,
+                    color = MaterialTheme.colorScheme.onSurfaceVariant)
+            }
+        }
     }
 }