Apply technical-light design system to the gateway dashboard

Restyles the Blazor dashboard onto a portable token-based theme so it
reads like an instrument panel: warm-paper background, hairline-ruled
panels, IBM Plex type, monospace tabular numerics, and status carried by
colour chips. Vendors theme.css + IBM Plex fonts, rewrites dashboard.css
as a thin token-driven view layer, and swaps the Bootstrap navbar and
status badges for the design-system app bar and chips.

Also includes pending API-key management, Galaxy hierarchy projection,
and constraint-enforcement work with their tests.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/MxGateway.Server/Security/Authentication/ApiKeyConstraintSerializer.cs

@@ -0,0 +1,28 @@
+using System.Text.Json;
+
+namespace MxGateway.Server.Security.Authentication;
+
+public static class ApiKeyConstraintSerializer
+{
+    private static readonly JsonSerializerOptions JsonOptions = new()
+    {
+        PropertyNamingPolicy = JsonNamingPolicy.SnakeCaseLower,
+        WriteIndented = false,
+    };
+
+    public static string? Serialize(ApiKeyConstraints constraints)
+    {
+        ArgumentNullException.ThrowIfNull(constraints);
+        return constraints.IsEmpty ? null : JsonSerializer.Serialize(constraints, JsonOptions);
+    }
+
+    public static ApiKeyConstraints Deserialize(string? json)
+    {
+        if (string.IsNullOrWhiteSpace(json))
+        {
+            return ApiKeyConstraints.Empty;
+        }
+
+        return JsonSerializer.Deserialize<ApiKeyConstraints>(json, JsonOptions) ?? ApiKeyConstraints.Empty;
+    }
+}