ux: eliminate white-flash on Blazor navigation from Inquiry page

- App.razor: loading overlay starts with `show` class (visible on cold load)
- admin-session.js: add showLoading()/hideLoading(); MutationObserver detects
  .admin-page-hero / .admin-login-page instead of mud-element count threshold;
  observer restarts on every navigation cycle via LocationChanged
- MainLayout.razor: subscribe to NavigationManager.LocationChanged →
  call JS showLoading() on every route change; implements IDisposable
- InquiryList.razor: remove unused IInquiryRepository injection; load data
  once (GetPagedAsync(1,200)) and pass IReadOnlyList to all six tab panels
- InquiryTable.razor: accept Inquiries parameter; filter synchronously in
  OnParametersSet() — eliminates 6 redundant API calls per page visit
- admin.css: overlay fade-in animation (0.15s); page content fade-in on
  route mount via .admin-page-hero / .admin-login-page animation (0.25s)

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

TaxBaik.Web/Components/Admin/App.razor

@@ -24,7 +24,7 @@
             <span>배포 또는 서버 재시작 중이면 잠시 후 자동으로 새로고침됩니다.</span>
         </div>
     </div>
-    <div id="blazor-loading" class="blazor-loading-overlay">
+    <div id="blazor-loading" class="blazor-loading-overlay show">
         <div class="blazor-loading-spinner">
             <div class="spinner"></div>
             <p>로드 중...</p>