feat(auth): implement option 3 architecture - server InteractiveServer login + client WASM dashboard

- Add Server.AddInteractiveServerComponents() + AddInteractiveServerRenderMode()
- Create Server AuthLayout for login form (MudBlazor)
- Implement LoginSimple.razor with @rendermode InteractiveServer in Server project
- Update App.razor: CascadingAuthenticationState + Router with AppAssembly=Server
- Fix Client MudBlazor Providers in MainLayout + AuthLayout
- Update Playwright tests: use dynamic selectors for MudTextField (auto-generated IDs)
- Build: 0 errors, 0 warnings
- API: /api/auth/login fully operational (200 OK confirmed)
- Playwright E2E test: 1 passed

Architecture:
/login    → Server InteractiveServer (MudBlazor form)
/         → Client WebAssembly (Dashboard)
/api/*    → Server Endpoints

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

src/dotnet/QuantEngine.Web/Components/Layout/AuthLayout.razor

@@ -0,0 +1,46 @@
+@inherits LayoutComponentBase
+@using QuantEngine.Web.Client.Theme
+
+<!-- Server-side Auth Layout for InteractiveServer login -->
+<MudThemeProvider Theme="@_theme" />
+<MudPopoverProvider />
+<MudDialogProvider />
+<MudSnackbarProvider />
+
+<div class="auth-container">
+    <div class="auth-right-panel">
+        <div class="auth-content">
+            @Body
+        </div>
+    </div>
+</div>
+
+<style>
+    .auth-container {
+        min-height: 100vh;
+        display: flex;
+        align-items: center;
+        justify-content: center;
+        background:
+            radial-gradient(circle at top left, rgba(0, 242, 254, 0.08), transparent 30%),
+            radial-gradient(circle at bottom right, rgba(79, 172, 254, 0.1), transparent 35%),
+            linear-gradient(135deg, #090a15 0%, #12142d 100%);
+    }
+
+    .auth-right-panel {
+        width: 100%;
+        display: flex;
+        align-items: center;
+        justify-content: center;
+        padding: 20px;
+    }
+
+    .auth-content {
+        width: 100%;
+        max-width: 480px;
+    }
+</style>
+
+@code {
+    private MudTheme _theme = AppTheme.LightTheme;
+}