Add biometric lock and rate limit handling

Biometric lock: opt-in Face ID/Touch ID/fingerprint app lock with toggle
in ProfileScreen. Locks on background, requires auth on foreground return.
Platform implementations: BiometricPrompt (Android), LAContext (iOS).

Rate limit: 429 responses parsed with Retry-After header, user-friendly
error messages in all 10 locales, retry plugin respects 429.
ErrorMessageParser updated for both iOS Swift and KMM.

composeApp/src/iosMain/kotlin/com/tt/honeyDue/MainViewController.kt

@@ -5,6 +5,8 @@ import com.tt.honeyDue.storage.TokenManager
 import com.tt.honeyDue.storage.TokenStorage
 import com.tt.honeyDue.storage.TaskCacheManager
 import com.tt.honeyDue.storage.TaskCacheStorage
+import com.tt.honeyDue.storage.BiometricPreference
+import com.tt.honeyDue.storage.BiometricPreferenceManager
 import com.tt.honeyDue.storage.ThemeStorage
 import com.tt.honeyDue.storage.ThemeStorageManager
 import com.tt.honeyDue.ui.theme.ThemeManager
@@ -20,5 +22,8 @@ fun MainViewController() = ComposeUIViewController {
     ThemeStorage.initialize(ThemeStorageManager.getInstance())
     ThemeManager.initialize()
 
+    // Initialize BiometricPreference storage
+    BiometricPreference.initialize(BiometricPreferenceManager.getInstance())
+
     App()
 }

composeApp/src/iosMain/kotlin/com/tt/honeyDue/platform/BiometricAuth.ios.kt

@@ -0,0 +1,57 @@
+package com.tt.honeyDue.platform
+
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.remember
+import kotlinx.cinterop.ExperimentalForeignApi
+import platform.LocalAuthentication.LAContext
+import platform.LocalAuthentication.LAPolicyDeviceOwnerAuthenticationWithBiometrics
+import kotlinx.coroutines.MainScope
+import kotlinx.coroutines.launch
+
+/**
+ * iOS implementation of biometric authentication using LocalAuthentication framework (LAContext).
+ */
+@OptIn(ExperimentalForeignApi::class)
+class IOSBiometricAuthPerformer : BiometricAuthPerformer {
+
+    override fun isBiometricAvailable(): Boolean {
+        val context = LAContext()
+        return context.canEvaluatePolicy(
+            LAPolicyDeviceOwnerAuthenticationWithBiometrics,
+            error = null
+        )
+    }
+
+    override fun authenticate(
+        title: String,
+        subtitle: String,
+        onResult: (BiometricResult) -> Unit
+    ) {
+        if (!isBiometricAvailable()) {
+            onResult(BiometricResult.NotAvailable)
+            return
+        }
+
+        val context = LAContext()
+        context.localizedFallbackTitle = ""
+
+        context.evaluatePolicy(
+            LAPolicyDeviceOwnerAuthenticationWithBiometrics,
+            localizedReason = subtitle
+        ) { success, error ->
+            MainScope().launch {
+                if (success) {
+                    onResult(BiometricResult.Success)
+                } else {
+                    val message = error?.localizedDescription ?: "Authentication failed"
+                    onResult(BiometricResult.Failed(message))
+                }
+            }
+        }
+    }
+}
+
+@Composable
+actual fun rememberBiometricAuth(): BiometricAuthPerformer {
+    return remember { IOSBiometricAuthPerformer() }
+}

composeApp/src/iosMain/kotlin/com/tt/honeyDue/storage/BiometricPreferenceManager.ios.kt

@@ -0,0 +1,28 @@
+package com.tt.honeyDue.storage
+
+import platform.Foundation.NSUserDefaults
+
+/**
+ * iOS implementation of biometric preference storage using NSUserDefaults.
+ * Follows the same pattern as ThemeStorageManager.ios.kt.
+ */
+actual class BiometricPreferenceManager {
+    private val defaults = NSUserDefaults.standardUserDefaults
+
+    actual fun isBiometricEnabled(): Boolean {
+        return defaults.boolForKey(KEY_BIOMETRIC_ENABLED)
+    }
+
+    actual fun setBiometricEnabled(enabled: Boolean) {
+        defaults.setBool(enabled, forKey = KEY_BIOMETRIC_ENABLED)
+        defaults.synchronize()
+    }
+
+    companion object {
+        private const val KEY_BIOMETRIC_ENABLED = "biometric_enabled"
+
+        private val instance by lazy { BiometricPreferenceManager() }
+
+        fun getInstance(): BiometricPreferenceManager = instance
+    }
+}