feat: enable enter key to submit login form

**Enhancement:**
- Wrap login form in HTML <form> element with @onsubmit
- HTML form automatically treats Enter key as submit action
- No need for custom @onkeypress handler

**Behavior:**
- Users can now press Enter in password field to login
- Or click the login button (existing behavior maintained)
- Both methods trigger HandleLogin() async handler

This provides better UX for keyboard-first users.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

TaxBaik.Web/Components/Admin/Pages/Login.razor

@@ -13,7 +13,7 @@
     <MudPaper Class="pa-8" Elevation="3" Style="width: 100%; max-width: 400px;">
         <MudText Typo="Typo.h4" Class="mb-6 text-center">관리자 로그인</MudText>
 
-        <div>
+        <form @onsubmit="HandleLogin" @onsubmit:preventDefault>
             <InputText class="mud-input mud-input-outlined mud-input-root mud-input-root-adorned-start mb-4"
                        style="width: 100%; min-height: 56px; padding: 16px 14px;"
                        placeholder="사용자명"
@@ -32,10 +32,9 @@
                 <MudAlert Severity="Severity.Error" Class="mb-4">@errorMessage</MudAlert>
             }
 
-            <button type="button"
+            <button type="submit"
                     class="mud-button-root mud-button mud-button-filled mud-button-filled-primary mud-elevation-0"
                     style="width: 100%; min-height: 52px; border: 0; border-radius: 4px; color: white;"
-                    @onclick="HandleLogin"
                     disabled="@isLoading">
                 @if (isLoading)
                 {
@@ -47,7 +46,7 @@
                     <span>로그인</span>
                 }
             </button>
-        </div>
+        </form>
     </MudPaper>
 </MudContainer>