fix: implement pure Blazor native login form for reliable auth state sync

Problem: With prerender: true + JavaScript form submission + location.reload(),
WASM hydration wasn't completing fast enough after page reload, leaving the
user on the login page despite successful token storage.

Solution: Complete rewrite to pure Blazor native login (prerender: false).
This approach:
1. WASM boots and renders the form
2. User submits form (Blazor handles it)
3. HttpClient POST to /api/auth/login
4. Save tokens to localStorage
5. CustomAuthenticationStateProvider.LoginAsync() called directly in C#
6. Blazor detects auth state change synchronously
7. NavigateTo() redirects to dashboard
8. All in same Blazor context, no reload needed

Benefits:
- Auth state update is synchronous with login response
- No WASM boot race conditions
- Direct C# call to CustomAuthenticationStateProvider
- Blazor handles redirect after auth state is confirmed

Trade-off: Login page requires WASM boot (brief spinner) instead of immediate
prerender display. This is acceptable for better reliability.

Result: Reliable login-to-dashboard flow with no hanging spinners or 'loading'
states.

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: true))
+@rendermode @(new InteractiveWebAssemblyRenderMode(prerender: false))
 <PageTitle>로그인</PageTitle>
-<AdminLoginForm />
+<AdminLoginFormNative />