feat: Convert website from static HTML to .NET 9.0 Blazor Web App

Major architectural upgrade from static HTML site to modern Blazor Web App:

- Migrated to .NET 9.0 Blazor Web App framework
- Converted home page with 4 gateway cards (Help Desk, App Store, Cloud, SDK)
- Added new SDK card linking to comprehensive SDK documentation
- Converted SDK documentation page to Blazor component
- Updated template download links to nuget.silverlabs.uk repository
- Implemented multi-stage Docker build with .NET SDK 9.0
- Created Blazor-optimized nginx configuration
- Preserved all original styling and animations
- Added .gitignore for Blazor build artifacts

Technical changes:
- New BlazorApp/ project structure with Components architecture
- MainLayout simplified (no default navigation)
- CSS ported to wwwroot (styles.css + sdk-styles.css)
- Multi-stage Dockerfile: Build with dotnet SDK, serve with nginx
- GitLab CI/CD pipeline compatible (auto-detects new Dockerfile)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

BlazorApp/Components/Pages/Error.razor

@@ -0,0 +1,36 @@
+@page "/Error"
+@using System.Diagnostics
+
+<PageTitle>Error</PageTitle>
+
+<h1 class="text-danger">Error.</h1>
+<h2 class="text-danger">An error occurred while processing your request.</h2>
+
+@if (ShowRequestId)
+{
+    <p>
+        <strong>Request ID:</strong> <code>@RequestId</code>
+    </p>
+}
+
+<h3>Development Mode</h3>
+<p>
+    Swapping to <strong>Development</strong> environment will display more detailed information about the error that occurred.
+</p>
+<p>
+    <strong>The Development environment shouldn't be enabled for deployed applications.</strong>
+    It can result in displaying sensitive information from exceptions to end users.
+    For local debugging, enable the <strong>Development</strong> environment by setting the <strong>ASPNETCORE_ENVIRONMENT</strong> environment variable to <strong>Development</strong>
+    and restarting the app.
+</p>
+
+@code{
+    [CascadingParameter]
+    private HttpContext? HttpContext { get; set; }
+
+    private string? RequestId { get; set; }
+    private bool ShowRequestId => !string.IsNullOrEmpty(RequestId);
+
+    protected override void OnInitialized() =>
+        RequestId = Activity.Current?.Id ?? HttpContext?.TraceIdentifier;
+}