fix: change domain from dev to lan to avoid browser HSTS preload blocking HTTP

The .dev TLD has been HSTS preloaded in Chrome/Firefox/Safari/Edge since 2019.
Browsers silently redirect http://anything.dev to https://anything.dev before
making any network request. Since Caddy has auto_https off, all browser-based
access to .dev domains fails with a connection error even though DNS, routing,
and HTTP all work correctly (curl works; browsers don't).

- cell_config.json: domain "dev" -> "lan"
- Caddyfile: all http://*.dev blocks -> http://*.lan
- Corefile: dev zone -> lan zone (file /data/lan.zone)
- data/dns/lan.zone: new zone file (dev.zone removed live)
- test_wg_domain_access.py: remove hardcoded DOMAIN_IPS / .dev references;
  read domain from /api/config at runtime so tests work with any configured TLD

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

config/dns/Corefile

@@ -5,8 +5,8 @@
     health
 }
 
-dev {
-    file /data/dev.zone
+lan {
+    file /data/lan.zone
     log
 }