feat: standalone Blazor WebAssembly admin + SEO enhancements

Architecture:
- Admin UI: /admin (Standalone Blazor WebAssembly, 219 WASM files)
- Portal: /portal (Razor Pages, Cookie/OAuth auth)
- Homepage: / (Razor Pages, SSR)
- API: /api (FastEndpoints + JWT)

SEO:
- Sitemap: Public content only (blog, FAQ, announcements, contact)
- robots.txt: Exclude /admin and /portal, reference production domain
- Naver verification: naverb1813cd79ddc2ded5c5291fca5cb46c2.html ready

Technical:
- TaxBaik.Web.Client: StaticWebAssetBasePath=admin
- Server Program.cs: UseBlazorFrameworkFiles + MapFallback for SPA routing
- base href="/admin/" for client-side navigation
- blazor.webassembly.js (standalone, not web.js)

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

src/TaxBaik.Web.Client/Components/Admin/Shared/AdminMetricCard.razor

@@ -0,0 +1,36 @@
+<div class="admin-metric-card @Accent cursor-pointer" @onclick="OnClick">
+    <div class="admin-metric-card-body">
+        <span class="admin-metric-card-label">@Label</span>
+        <div class="admin-metric-card-value-row">
+            <span class="admin-metric-card-value" style="color: @ValueColor;">@Value</span>
+            <span class="admin-metric-card-icon" style="color: @IconColor;">@Icon</span>
+        </div>
+        <span class="admin-metric-card-caption">@Caption</span>
+    </div>
+</div>
+
+@code {
+    [Parameter, EditorRequired]
+    public string Label { get; set; } = "";
+
+    [Parameter, EditorRequired]
+    public object? Value { get; set; }
+
+    [Parameter, EditorRequired]
+    public string Caption { get; set; } = "";
+
+    [Parameter, EditorRequired]
+    public string Accent { get; set; } = "";
+
+    [Parameter, EditorRequired]
+    public string Icon { get; set; } = "";
+
+    [Parameter]
+    public string ValueColor { get; set; } = "inherit";
+
+    [Parameter]
+    public string IconColor { get; set; } = "inherit";
+
+    [Parameter]
+    public EventCallback OnClick { get; set; }
+}