fix: implement Blazor-native login form to properly update authentication state

Problem: JavaScript login form saved tokens to localStorage but didn't notify
CustomAuthenticationStateProvider, causing [Authorize] pages to remain in
'loading' state indefinitely. The provider only reads tokens when:
1. GetAuthenticationStateAsync() is called (page load)
2. NotifyAuthenticationStateChanged() is triggered (UI updates)

But JavaScript login didn't trigger either, leaving the authentication state
stale.

Solution: Convert AdminLoginForm from HTML+JavaScript to pure Blazor component.
Now the login flow is:
1. User enters credentials in Blazor form
2. HttpClient POST to /api/auth/login
3. Save tokens to localStorage
4. Call CustomAuthenticationStateProvider.LoginAsync() directly
5. Blazor detects auth state change and re-evaluates [Authorize] pages
6. Dashboard [Authorize] page renders successfully

Result: Immediate authentication state update, no loading timeout on protected pages.

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

src/TaxBaik.Web/wwwroot/js/admin-session.js

@@ -341,6 +341,9 @@ window.taxbaikAdminSession = {
                     localStorage.removeItem('admin-remember-checkbox');
                 }
 
+                // 토큰을 저장한 직후 페이지 리로드
+                // 이렇게 하면 CustomAuthenticationStateProvider의 GetAuthenticationStateAsync()가
+                // localStorage에서 토큰을 복원하고 [Authorize] 페이지가 제대로 렌더링됨
                 window.location.href = '/taxbaik/admin/dashboard';
             } catch (error) {
                 window.taxbaikAdminSession.traceUiState('admin-login', `submit failed: ${error?.message || 'login failed'}`);