feat: add dnsmasq DNS record admin UI

- store records in SQLite via built-in node:sqlite, no native deps
- wrap DB reads in await io() so they stay out of the static shell
- add CRUD pages backed by Server Actions with per-request data
- generate managed config and restart container via Docker socket
- add /resolve page to query dnsmasq for A and CNAME records
- add Dockerfile and compose setup with shared config volume

lib/env.ts

@@ -0,0 +1,26 @@
+import path from "node:path"
+
+function withDefault(name: string, fallback: string): string {
+  const raw = process.env[name]
+  return raw && raw.trim() ? raw.trim() : fallback
+}
+
+function toAbsolute(target: string): string {
+  return path.isAbsolute(target) ? target : path.resolve(process.cwd(), target)
+}
+
+/**
+ * Runtime configuration. Everything is optional with sensible defaults so the app can
+ * boot locally without any env file; see `.env.example` for the deploy-time values.
+ */
+export const env = {
+  /** SQLite database file — the single source of truth for records. */
+  dbPath: toAbsolute(withDefault("DNSMASQ_ADMIN_DB_PATH", "./data/dnsmasq-admin.db")),
+  /** Managed dnsmasq config file that this app regenerates on every apply. */
+  confPath: toAbsolute(withDefault("DNSMASQ_ADMIN_CONF_PATH", "./data/dnsmasq-admin.conf")),
+  /** Container that gets restarted through the Docker socket after an apply. */
+  containerName: withDefault("DNSMASQ_CONTAINER_NAME", "dnsmasq"),
+  dockerSocket: withDefault("DNSMASQ_ADMIN_DOCKER_SOCKET", "/var/run/docker.sock"),
+  /** DNS server that the /resolve page queries. */
+  dnsServer: withDefault("DNSMASQ_SERVER_IP", "127.0.0.1"),
+}