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/build.gradle.kts

@@ -71,6 +71,10 @@ kotlin {
 
             // Encrypted SharedPreferences for secure token storage
             implementation(libs.androidx.security.crypto)
+
+            // Biometric authentication (requires FragmentActivity)
+            implementation("androidx.biometric:biometric:1.1.0")
+            implementation("androidx.fragment:fragment-ktx:1.8.5")
         }
         iosMain.dependencies {
             implementation(libs.ktor.client.darwin)