feat: implement M3 — UDP/TCP DNS listener

- Add DnsListener that binds UDP and TCP on configurable port
- Add [dns] config section with listen address (default 0.0.0.0:5353)
- DNS over TCP uses 2-byte length prefix (RFC 1035 §4.2.2)
- Forward queries through existing DoH pipeline
- Start DNS listener as background task alongside DoH server

config/default.toml

@@ -1,6 +1,9 @@
 [server]
 listen = "0.0.0.0:8800"
 
+[dns]
+listen = "0.0.0.0:5353"
+
 [upstream]
 resolvers = [
     { name = "cloudflare", url = "https://cloudflare-dns.com/dns-query" },