fix: restore Blazor WebAssembly render mode for ERP scalability

Restore long-term architectural goal: Blazor WebAssembly for admin UI.

Rationale:
- TaxBaik is a semi-project for future ERP implementation
- ERP requires client-side scalability (no server-side state per user)
- WebAssembly offloads interactivity to browser (Circuit-free)
- Aligns with API-First + stateless backend design

Changes:
- App.razor: InteractiveWebAssemblyRenderMode (prerender: true)
- Routes: InteractiveWebAssemblyRenderMode (prerender: true)
- Login.razor: InteractiveWebAssemblyRenderMode (prerender: true)
- Program.cs: AddInteractiveWebAssemblyComponents()
- Updated validation script to enforce WebAssembly mode

Tradeoff accepted: Blazor WebAssembly bootstrap time vs future ERP extensibility.

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

TaxBaik.Web/Components/Admin/Pages/Login.razor

@@ -1,6 +1,6 @@
 @page "/admin/login"
 @layout TaxBaik.Web.Components.Admin.Layout.BlankLayout
 @attribute [AllowAnonymous]
-@rendermode @(new InteractiveServerRenderMode(prerender: true))
+@rendermode @(new InteractiveWebAssemblyRenderMode(prerender: true))
 <PageTitle>로그인</PageTitle>
 <AdminLoginForm />