Add dnsmasq version awareness, readiness endpoint, and Docker healthcheck

- Options: VersionCommand, VersionTimeoutSeconds, MinimumVersion, EnforceMinimumVersion
- Version probe service and parser; startup enforcement when EnforceMinimumVersion
- Save flow: block save on probe failure (VersionProbeFailed) or version below minimum (UnsupportedVersion)
- Status API and Dnsmasq page: show version with tooltip, link to CHANGELOG
- Readiness: GET /healthz/ready (version + dnsmasq running when StatusCommand set)
- MapReadyHealthCheck extension; Dockerfile HEALTHCHECK + curl; compose healthcheck and service_healthy
- Docs: README and agents.md updated

src/DnsmasqWebUI/Components/Pages/Dnsmasq.razor

@@ -24,6 +24,22 @@ else if (_error != null)
 else if (_status != null)
 {
     <section class="page-section">
+        @if (!string.IsNullOrWhiteSpace(_status.DnsmasqVersion) || !string.IsNullOrWhiteSpace(_status.DnsmasqVersionError))
+        {
+            <p class="text-muted small mb-1 text-end">
+                @if (!string.IsNullOrWhiteSpace(_status.DnsmasqVersion))
+                {
+                    var versionTooltip = "Minimum required by this UI: " + _status.MinimumSupportedDnsmasqVersion + ". " + (_status.DnsmasqVersionSupported ? "Supported." : "Unsupported.");
+                    <a href="https://thekelleys.org.uk/dnsmasq/CHANGELOG" target="_blank" rel="noopener noreferrer" class="text-muted text-decoration-none" title="@versionTooltip">
+                        dnsmasq <code>@_status.DnsmasqVersion</code>
+                    </a>
+                }
+                @if (!string.IsNullOrWhiteSpace(_status.DnsmasqVersionError))
+                {
+                    <span class="text-warning ms-1" title="@_status.DnsmasqVersionError">@_status.DnsmasqVersionError</span>
+                }
+            </p>
+        }
         <h2 class="page-section-title">Service configuration</h2>
         <p class="mb-1"><strong>Config files (load order):</strong></p>
         <ul class="list-unstyled small ms-2 mb-0">