feat: add message content length validation

- Backend: MinMessageLength=10, MaxMessageLength=5000
- Frontend: Real-time character counter
- Frontend: Client-side validation before submission
- Frontend: Error messages for length violations
- Applied to both Submit and Update operations

Prevents empty or excessively long messages while maintaining
user-friendly feedback on character count.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/TaxBaik.Web/Pages/Contact.cshtml

@@ -72,7 +72,11 @@
 
         <div class="mb-3">
             <label for="message" class="form-label">문의내용 <span class="text-danger">*</span></label>
-            <textarea class="form-control" id="message" name="Message" rows="5" required></textarea>
+            <textarea class="form-control" id="message" name="Message" rows="5" required minlength="10" maxlength="5000" placeholder="최소 10자, 최대 5000자까지 입력 가능합니다"></textarea>
+            <small class="form-text text-muted">
+                <span id="messageCount">0</span>/5000
+            </small>
+            <div id="messageError" class="text-danger mt-2" style="display: none;"></div>
         </div>
 
         <div class="mb-3 form-check">
@@ -88,8 +92,14 @@
     <script>
         const phoneInput = document.getElementById('phone');
         const phoneError = document.getElementById('phoneError');
+        const messageInput = document.getElementById('message');
+        const messageError = document.getElementById('messageError');
+        const messageCount = document.getElementById('messageCount');
         const contactForm = document.getElementById('contactForm');
 
+        const MIN_MESSAGE_LENGTH = 10;
+        const MAX_MESSAGE_LENGTH = 5000;
+
         // 한국 전화번호 정규식
         const koreanPhoneRegex = /^(0(2|3[1-3]|4[1-4]|5[1-5]|6[1-4]|70|50[5-9]|[7-9](?:\d{1,2})?)\d{7,8}|0\d{9,10})$/;
 
@@ -161,6 +171,47 @@
 
         // 포커스 아웃 시 최종 검증
         phoneInput.addEventListener('blur', validatePhone);
+
+        // 메시지 길이 실시간 표시
+        messageInput.addEventListener('input', (e) => {
+            const length = e.target.value.length;
+            messageCount.textContent = length;
+            validateMessage();
+        });
+
+        // 메시지 검증
+        function validateMessage() {
+            const value = messageInput.value.trim();
+            const isValid = value.length >= MIN_MESSAGE_LENGTH && value.length <= MAX_MESSAGE_LENGTH;
+
+            if (!isValid && messageInput.value.length > 0) {
+                messageError.style.display = 'block';
+                if (value.length < MIN_MESSAGE_LENGTH) {
+                    messageError.textContent = `최소 ${MIN_MESSAGE_LENGTH}자 이상 입력해주세요. (현재: ${value.length}자)`;
+                } else if (value.length > MAX_MESSAGE_LENGTH) {
+                    messageError.textContent = `최대 ${MAX_MESSAGE_LENGTH}자까지만 입력 가능합니다. (현재: ${value.length}자)`;
+                }
+                messageInput.classList.add('is-invalid');
+            } else {
+                messageError.style.display = 'none';
+                messageInput.classList.remove('is-invalid');
+            }
+
+            return isValid;
+        }
+
+        // 폼 제출 시 메시지 검증 포함
+        const originalSubmitHandler = contactForm.onsubmit;
+        contactForm.addEventListener('submit', (e) => {
+            if (!validatePhone() || !validateMessage()) {
+                e.preventDefault();
+                if (!validatePhone()) {
+                    phoneInput.focus();
+                } else if (!validateMessage()) {
+                    messageInput.focus();
+                }
+            }
+        });
     </script>
 
     <hr class="my-5" />