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.

iosApp/HoneyDueTests/ErrorMessageParserTests.swift

@@ -48,6 +48,11 @@ struct ErrorCodeMappingTests {
         #expect(result == "Too many attempts. Please wait a few minutes and try again.")
     }
 
+    @Test func tooManyRequestsCode() {
+        let result = ErrorMessageParser.parse("error.too_many_requests")
+        #expect(result == "Too many requests. Please try again later.")
+    }
+
     @Test func taskNotFoundCode() {
         let result = ErrorMessageParser.parse("error.task_not_found")
         #expect(result == "Task not found. It may have been deleted.")
@@ -137,6 +142,16 @@ struct NetworkErrorPatternTests {
         let result = ErrorMessageParser.parse("SocketTimeoutException: connect timed out")
         #expect(result == "Request timed out. Please try again.")
     }
+
+    @Test func tooManyRequestsDetected() {
+        let result = ErrorMessageParser.parse("Too many requests. Please try again later.")
+        #expect(result == "Too many requests. Please try again later.")
+    }
+
+    @Test func tooManyRequestsWithRetryAfterDetected() {
+        let result = ErrorMessageParser.parse("Too many requests. Please try again in 30 seconds.")
+        #expect(result.contains("Too many requests"))
+    }
 }
 
 // MARK: - Technical Error Detection Tests