fix: restore HTML login form with prerender: true per spec requirements

Context: Validation script requires Login.razor to use prerender: true for
immediate form display before WASM boots (Phase 9 requirement).

Solution: Revert to original HTML form + JavaScript approach:
- AdminLoginForm: HTML form (statically rendered, works with prerender: true)
- admin-session.js: JavaScript login handler
- Post-login: 200ms delay before redirect to allow CustomAuthenticationStateProvider
  to read tokens from localStorage and establish auth state

Flow:
1. User submits form (JavaScript handles it)
2. POST /api/auth/login
3. Save tokens to localStorage
4. 200ms delay
5. Redirect to /taxbaik/admin/dashboard
6. Page loads with Blazor bootstrapping + auth state restored

Result: Login form displays immediately (prerender: true) while maintaining
proper authentication state propagation for [Authorize] pages.

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

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

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