feat: DDNS settings integration — check availability, update credentials

- GET /api/config now returns domain_mode, domain_name, ddns.{provider,subdomain,has_token}
- GET /api/ddns/check/<name> proxies availability check to DDNS service
- PUT /api/ddns validates and saves cloudflare/duckdns credentials post-setup
- When cell_name changes for pic_ngo provider, auto-registers the new subdomain
- Settings: Cell Name shows availability badge for pic_ngo; auto-save blocks on taken
- Settings: new External Domain & DDNS section — pic_ngo info, cloudflare/duckdns edit
- 11 new tests for the two new endpoints (all pass)

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

webui/src/services/api.js

@@ -321,6 +321,12 @@ export const logsAPI = {
   setVerbosity: (levels) => api.put('/api/logs/verbosity', levels),
 };
 
+// DDNS API
+export const ddnsAPI = {
+  checkName: (name) => api.get(`/api/ddns/check/${name}`),
+  updateConfig: (data) => api.put('/api/ddns', data),
+};
+
 // Setup Wizard API
 export const setupAPI = {
   getStatus: () => api.get('/api/setup/status'),