fix: correct phone number formatting to follow standard pattern

- 10-digit numbers: XXXX-XXX-XXXX (4-3-3) - landline format
- 11-digit numbers: XXX-XXXX-XXXX (3-4-4) - mobile format

Apply consistent formatting on both frontend and backend.

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

src/TaxBaik.Web/Pages/Contact.cshtml

@@ -98,11 +98,13 @@
                 value = value.substring(0, 11); // 최대 11자리
             }
 
-            // 포맷팅: XXX-XXXX-XXXX 또는 XXX-XXX-XXXX
+            // 포맷팅
             if (value.length >= 10) {
                 if (value.length === 10) {
-                    value = `${value.substring(0, 3)}-${value.substring(3, 6)}-${value.substring(6)}`;
+                    // 10자리: XXXX-XXX-XXXX
+                    value = `${value.substring(0, 4)}-${value.substring(4, 7)}-${value.substring(7)}`;
                 } else if (value.length === 11) {
+                    // 11자리: XXX-XXXX-XXXX
                     value = `${value.substring(0, 3)}-${value.substring(3, 7)}-${value.substring(7)}`;
                 }
             } else if (value.length > 3) {