Initial commit: dnsmasq web UI

- ASP.NET Core 9 Blazor Web App + Web API
- Hosts file management (read/write, hosts(5) format)
- DHCP static hosts (dhcp-host=, dnsmasq config)
- DHCP leases (read-only, dnsmasq lease file format)
- Reload and status commands (configurable)
- Docker: standalone app image + Dockerfile.dnsmasq (app + dnsmasq)
- docker-compose.test.yml: app + dhcp-client, testdata mount
- Unit tests (Hosts, DhcpHost, Leases, DnsmasqConfig parsers) using testdata

entrypoint.sh

@@ -0,0 +1,7 @@
+#!/bin/sh
+# If DNSMASQ_CONF is set, run dnsmasq in the background (same container as the app).
+# Container main process is the app (exec below).
+if [ -n "$DNSMASQ_CONF" ]; then
+  dnsmasq -k --conf-file="$DNSMASQ_CONF" &
+fi
+exec dotnet DnsmasqWebUI.dll