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/commonMain/composeResources/values-zh/strings.xml

@@ -421,6 +421,8 @@
     <string name="error_network">网络错误，请检查您的连接。</string>
     <string name="error_timeout">请求超时，请重试。</string>
     <string name="error_server">服务器错误，请稍后重试。</string>
+    <string name="error_rate_limit">请求过多，请稍后重试。</string>
+    <string name="error_rate_limit_with_delay">请求过多，请在%1$d秒后重试。</string>
     <string name="error_unauthorized">会话已过期，请重新登录。</string>
     <string name="error_not_found">未找到</string>
     <string name="error_invalid_input">请检查您的输入</string>