Refactor: reorganise namespaces and move service DTOs to Models/Contracts

- Move Client/Http, Services, Parsers under Infrastructure/ (Client, Services, Parsers)
- Move config helpers (DnsmasqConfKeys, DnsmasqOptionTooltips, DnsmasqFileEncoding, EffectiveConfigFieldBuilder) to Infrastructure/Helpers/Config; ApiJsonOptions to Helpers/Http
- Move Configuration types (ApplicationOptions, DnsmasqOptions, DnsmasqOptionsValidator) to Models/Config
- Merge Models/Status and Models/EffectiveConfig into Models/Dnsmasq and Models/Dnsmasq/EffectiveConfig
- Move Client/Models and Client/Services to Models/Client and Infrastructure/Services
- Move ConfigSetSnapshot, HostsSnapshot, ProcessRunResult, ManagedConfigContent from Infrastructure/Services/Abstractions to Models/Contracts
- Update all namespaces and usings; tests pass

src/DnsmasqWebUI/Infrastructure/Client/SameHostBaseAddressHandler.cs

@@ -0,0 +1,27 @@
+using System.Net;
+using DnsmasqWebUI.Infrastructure.Services.Abstractions;
+
+namespace DnsmasqWebUI.Infrastructure.Client;
+
+/// <summary>
+/// DelegatingHandler that rewrites relative request URIs to the current request's scheme, host, and path base.
+/// Used so Blazor/API callers can use relative paths (e.g. "/api/status") and hit the same host.
+/// Resolved in the same scope as the code that requested the HttpClient (e.g. Blazor component),
+/// so IHttpContextAccessor has the current request when available.
+/// </summary>
+public sealed class SameHostBaseAddressHandler(IHttpContextAccessor httpContextAccessor) : DelegatingHandler, IApplicationScopedConcrete
+{
+    protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
+    {
+        var context = httpContextAccessor.HttpContext;
+        if (context != null && request.RequestUri != null)
+        {
+            var baseUri = new Uri($"{context.Request.Scheme}://{context.Request.Host.Value}{context.Request.PathBase.Value ?? ""}");
+            request.RequestUri = request.RequestUri.IsAbsoluteUri
+                ? new Uri(baseUri, request.RequestUri.PathAndQuery)
+                : new Uri(baseUri, request.RequestUri);
+        }
+
+        return base.SendAsync(request, cancellationToken);
+    }
+}