feat: Migrate admin UI from Blazor WASM/MudBlazor to Razor Pages/Cookie Auth/Tabler

- Remove QuantEngine.Web.Client from .sln (keep on disk for reference)
- Replace Blazor Interactive WebAssembly with server-rendered Razor Pages
- Implement Cookie Authentication (HttpOnly, SameSite=Lax, 12h expiry)
- Add AuthService with BCrypt password hashing + auto-migration from SHA-256
- Implement IpLockoutService (3 strikes → 15-min ban)
- Create Admin folder structure with Layout + shared partials
- Implement Dashboard, Collection, Users index pages (base structure)
- Remove hardcoded backdoors (master_recovery, dev auth bypass)
- Remove hardcoded localhost:5265 URLs
- Add Tabler UI base styling (Bootstrap 5 CDN + custom admin.css)
- Update CLAUDE.md with new UI standards and auth policies
- Build: 0 errors, 0 warnings (ready for dev testing)

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

src/dotnet/QuantEngine.Web/Pages/Admin/Dashboard/Index.cshtml

@@ -0,0 +1,66 @@
+@page
+@model QuantEngine.Web.Pages.Admin.Dashboard.IndexModel
+@{
+    ViewData["Title"] = "대시보드";
+}
+
+<div class="page-header d-print-none">
+    <div class="row align-items-center">
+        <div class="col">
+            <h2 class="page-title">대시보드</h2>
+        </div>
+    </div>
+</div>
+
+<div class="page-body">
+    <div class="row row-deck row-cards">
+        <div class="col-md-6">
+            <div class="card">
+                <div class="card-body">
+                    <div class="text-truncate">
+                        <h3 class="card-title">활성 사용자</h3>
+                        <div class="h2 mt-3">@(Model.ActiveUsersCount ?? 0)</div>
+                    </div>
+                </div>
+            </div>
+        </div>
+        <div class="col-md-6">
+            <div class="card">
+                <div class="card-body">
+                    <div class="text-truncate">
+                        <h3 class="card-title">최근 수집 실행</h3>
+                        <div class="h2 mt-3">@(Model.RecentRunsCount ?? 0)</div>
+                    </div>
+                </div>
+            </div>
+        </div>
+    </div>
+
+    <div class="row row-deck row-cards mt-4">
+        <div class="col-12">
+            <div class="card">
+                <div class="card-header">
+                    <h3 class="card-title">최근 시스템 이벤트</h3>
+                </div>
+                <div class="table-responsive">
+                    <table class="table card-table table-vcenter">
+                        <thead>
+                            <tr>
+                                <th>시간</th>
+                                <th>이벤트</th>
+                                <th>상태</th>
+                            </tr>
+                        </thead>
+                        <tbody>
+                            <tr>
+                                <td>@DateTime.UtcNow.ToString("yyyy-MM-dd HH:mm")</td>
+                                <td>시스템 초기화</td>
+                                <td><span class="badge bg-success">완료</span></td>
+                            </tr>
+                        </tbody>
+                    </table>
+                </div>
+            </div>
+        </div>
+    </div>
+</div>

src/dotnet/QuantEngine.Web/Pages/Admin/Dashboard/Index.cshtml.cs

@@ -0,0 +1,40 @@
+using Microsoft.AspNetCore.Authorization;
+using Microsoft.AspNetCore.Mvc.RazorPages;
+using QuantEngine.Core.Interfaces;
+using QuantEngine.Web.Services;
+
+namespace QuantEngine.Web.Pages.Admin.Dashboard;
+
+[Authorize(AuthenticationSchemes = AdminAuthDefaults.Scheme)]
+public class IndexModel : PageModel
+{
+    private readonly IWorkspaceRepository _workspaceRepository;
+    private readonly ICollectionRepository _collectionRepository;
+    private readonly ILogger<IndexModel> _logger;
+
+    public int? ActiveUsersCount { get; set; }
+    public int? RecentRunsCount { get; set; }
+
+    public IndexModel(IWorkspaceRepository workspaceRepository, ICollectionRepository collectionRepository, ILogger<IndexModel> logger)
+    {
+        _workspaceRepository = workspaceRepository;
+        _collectionRepository = collectionRepository;
+        _logger = logger;
+    }
+
+    public async Task OnGetAsync()
+    {
+        try
+        {
+            var accounts = await _workspaceRepository.GetAccountsAsync();
+            ActiveUsersCount = accounts.Count(a => string.Equals(a.IsActive, "true", StringComparison.OrdinalIgnoreCase));
+
+            var dashboard = await _collectionRepository.GetDashboardStateAsync();
+            RecentRunsCount = string.IsNullOrEmpty(dashboard?.LastRunId) ? 0 : 1;
+        }
+        catch (Exception ex)
+        {
+            _logger.LogError(ex, "Dashboard data loading failed");
+        }
+    }
+}