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/Layout/MainLayout.razor

@@ -1,4 +1,7 @@
 @inherits LayoutComponentBase
+@inject NavigationManager Navigation
+@inject IJSRuntime JS
+@implements IDisposable
 
 <MudLayout Class="admin-shell">
     <MudAppBar Elevation="0" Class="admin-topbar">
@@ -73,8 +76,23 @@
     private bool expandedCustomerGroup = true;
     private bool expandedWebsiteGroup = false;
 
+    protected override void OnInitialized()
+    {
+        Navigation.LocationChanged += OnLocationChanged;
+    }
+
+    private void OnLocationChanged(object? sender, LocationChangedEventArgs args)
+    {
+        _ = InvokeAsync(() => JS.InvokeVoidAsync("taxbaikAdminSession.showLoading"));
+    }
+
     private void ToggleDrawer()
     {
         drawerOpen = !drawerOpen;
     }
+
+    public void Dispose()
+    {
+        Navigation.LocationChanged -= OnLocationChanged;
+    }
 }