fix: restore complete index.html and remove Portal fallback

- Restore admin/index.html with all required stylesheets and scripts:
  * MudBlazor CSS/JS
  * Material Icons
  * Google Fonts
  * Admin CSS
  * Loading spinner and error UI
- Remove unsupported Portal MapFallbackToFile that referenced non-existent portal/index.html

This fixes the incomplete page rendering (542 bytes → proper HTML size).

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

src/TaxBaik.Web/Program.cs

@@ -394,7 +394,6 @@ app.MapStaticAssets();
 
 // SPA 라우팅 폴백 (가장 마지막에!)
 app.MapFallbackToFile("admin/{*path:nonfile}", "admin/index.html");
-app.MapFallbackToFile("portal/{*path:nonfile}", "portal/index.html");
 
 // 애플리케이션 시작/종료 로깅
 try