headscale: fix config schema, DERP map, and UI routing; document setup

- config.yaml: migrate to current headscale config schema (database/dns
  nested keys instead of legacy db_type/db_path/dns_config), add noise
  private key path, prefixes, and a DERP map (required at startup)
- compose.yaml: bind-mount a fixed Caddyfile for headscale-ui so it
  serves from /app/admin instead of 404ing at /app
- README: document NPM reverse proxy setup (websockets on, HTTP/2 off
  to avoid breaking the ts2021 noise protocol handshake), split
  Android/iOS vs laptop client instructions, add headscale-ui admin
  panel setup with API key generation, switch CLI examples to
  docker exec against the running container

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

headscale_dev/config.yaml

@@ -1,12 +1,33 @@
 server_url: https://headscale.example.com
 listen_addr: 0.0.0.0:8080
 
-db_type: sqlite3
-db_path: /var/lib/headscale/db.sqlite
+noise:
+  private_key_path: /var/lib/headscale/noise_private.key
+
+database:
+  type: sqlite
+  sqlite:
+    path: /var/lib/headscale/db.sqlite
+
+prefixes:
+  v4: 100.64.0.0/10
+  v6: fd7a:115c:a1e0::/48
+
+derp:
+  server:
+    enabled: false
+  urls:
+    - https://controlplane.tailscale.com/derpmap/default
+  auto_update_enabled: true
+  update_frequency: 24h
 
 log:
   level: info
 
-dns_config:
+dns:
   override_local_dns: true
-  base_domain: tailnet.local
+  base_domain: tailnet.local
+  nameservers:
+    global:
+      - 1.1.1.1
+      - 8.8.8.8