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

testdata/README.md

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+# Test data for docker-compose.test.yml
+
+- **hosts** – Shared by app and dnsmasq (read/write by app, `addn-hosts` for dnsmasq).
+- **dnsmasq.conf** – Sample main config (reference only; not used by the test containers).
+- **dnsmasq-test.conf** – Config for dnsmasq when run inside the app container (DHCP-only, paths under `/data`).
+- **dnsmasq.d/dhcp.conf** – DHCP static hosts; app edits this, dnsmasq loads it via `conf-dir`.
+- **dnsmasq.d/01-other.conf** – Other snippet (not managed by app).
+- **leases** – Empty at start. dnsmasq creates and populates it when the dhcp-client container gets a lease. App reads it (read-only).
+
+All of this is bind-mounted as `/data` in the **app** container; the app runs as the main process and dnsmasq runs in the background in the same container.

testdata/dnsmasq-test.conf

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+# Config for the dnsmasq test container only (docker-compose.test.yml).
+# DHCP-only; uses same testdata layout so app and dnsmasq share hosts, dhcp.conf, leases.
+port=0
+interface=eth0
+bind-interfaces
+dhcp-range=172.28.0.10,172.28.0.50,255.255.255.0,12h
+dhcp-leasefile=/data/leases
+conf-dir=/data/dnsmasq.d
+addn-hosts=/data/hosts

testdata/dnsmasq.conf

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+# Sample main dnsmasq config (not managed by this app).
+# In production this would typically be /etc/dnsmasq.conf.
+# It includes all .conf files from dnsmasq.d/ (e.g. dhcp.conf managed by this app).
+conf-dir=/etc/dnsmasq.d/

testdata/dnsmasq.d/01-other.conf

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+# Not managed by this app. The app only edits dhcp.conf; this file should stay unchanged when saving from the UI.
+domain=local

testdata/dnsmasq.d/dhcp.conf

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+# Sample dhcp-host entries for integration testing (managed by this app)
+dhcp-host=aa:bb:cc:dd:ee:ff,192.168.1.10,testpc,infinite

testdata/hosts

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+127.0.0.1 localhost
+::1 localhost

testdata/leases

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+1738166400 aa:bb:cc:dd:ee:ff 192.168.1.10 testpc 01:02:03:04:05:06
+1738166300 11:22:33:44:55:66 192.168.1.11 * *