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>

tests/e2e/admin-login.spec.ts

@@ -9,6 +9,8 @@ test.describe('admin authentication', () => {
     test.skip(!password, 'E2E_ADMIN_PASSWORD is required.');
 
     const consoleErrors: string[] = [];
+    const networkRequests: { url: string; status?: number }[] = [];
+
     page.on('console', message => {
       if (message.type() === 'error') {
         consoleErrors.push(message.text());
@@ -17,16 +19,60 @@ test.describe('admin authentication', () => {
     page.on('pageerror', error => {
       consoleErrors.push(error.message);
     });
+    page.on('response', response => {
+      if (response.url().includes('/api/auth/login') || response.url().includes('/admin/')) {
+        networkRequests.push({ url: response.url(), status: response.status() });
+      }
+    });
 
     await page.goto(`${baseUrl}/admin/login`);
     await expect(page.locator('input[placeholder="사용자명"]')).toBeVisible();
     await expect(page.locator('input[placeholder="비밀번호"]')).toBeVisible();
 
-    await page.locator('input[placeholder="사용자명"]').fill(username);
-    await page.locator('input[placeholder="비밀번호"]').fill(password);
-    await page.getByRole('button', { name: '로그인' }).click();
+    // Wait for form to be fully ready
+    await page.waitForTimeout(500);
 
-    await expect(page).toHaveURL(/\/taxbaik\/admin\/dashboard$/);
+    const usernameInput = page.locator('input[placeholder="사용자명"]');
+    const passwordInput = page.locator('input[placeholder="비밀번호"]');
+    const loginButton = page.getByRole('button', { name: '로그인' });
+
+    await usernameInput.fill(username);
+    await passwordInput.fill(password);
+
+    // Wait for inputs to register
+    await page.waitForTimeout(300);
+
+    // Verify inputs were filled
+    const usernameValue = await usernameInput.inputValue();
+    const passwordValue = await passwordInput.inputValue();
+    console.log(`Username filled: ${usernameValue === username}, Password filled: ${passwordValue === password}`);
+
+    // Click login and wait for any navigation
+    await loginButton.click();
+
+    // Wait a bit for form submission + page reload
+    await page.waitForTimeout(3000);
+
+    // Check localStorage and current state
+    const localStorageData = await page.evaluate(() => ({
+      accessToken: localStorage.getItem('accessToken'),
+      refreshToken: localStorage.getItem('refreshToken'),
+      tokenExpiry: localStorage.getItem('tokenExpiry')
+    }));
+
+    console.log(`localStorage has accessToken: ${!!localStorageData.accessToken}`);
+    console.log(`Current URL after login: ${page.url()}`);
+    console.log(`Network requests: ${JSON.stringify(networkRequests)}`);
+
+    // If we're still on login page, something failed
+    if (page.url().includes('/admin/login')) {
+      console.log('Still on login page after 3 seconds!');
+      // Try to find error message
+      const errorMsg = await page.locator('.login-error-message').textContent().catch(() => null);
+      console.log(`Error message: ${errorMsg}`);
+    }
+
+    await expect(page).toHaveURL(/\/taxbaik\/admin\/dashboard$/, { timeout: 20_000 });
     await expect(page.getByRole('heading', { name: '대시보드' }).first()).toBeVisible({ timeout: 20_000 });
     await expect(page.getByRole('link', { name: /로그아웃/ })).toBeVisible();
     expect(consoleErrors, 'browser console/page errors').toEqual([]);