feat: improve setup wizard and DDNS UX

Setup wizard (Issue 1 — UI):
- pic.ngo subdomain input now uses the same split-field style as DuckDNS:
  input + static '.pic.ngo' suffix in a flex row, availability status below

Setup wizard (Issue 2 — Caddy not regenerating after completion):
- complete_setup route now fires IDENTITY_CHANGED after a successful wizard
  submission so CaddyManager regenerates the Caddyfile immediately; users
  no longer need to press 'Renew Certificate' to start ACME

Settings — DDNS status (Issue 2 — domain status missing):
- New GET /api/ddns/status endpoint: returns registered flag, domain_name,
  public_ip (ipify with 30s cache), last_ip from heartbeat
- Settings DDNS section for pic_ngo now shows a live status row with
  color-coded dot (green=registered+current, yellow=registered+stale,
  gray=not registered), current public IP, and a Check button
- Status auto-refreshes on mount and after each successful re-registration

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

webui/src/services/api.js

@@ -343,6 +343,7 @@ export const ddnsAPI = {
   checkName: (name) => api.get(`/api/ddns/check/${name}`),
   updateConfig: (data) => api.put('/api/ddns', data),
   register: () => api.post('/api/ddns/register'),
+  getStatus: () => api.get('/api/ddns/status'),
 };
 
 // Setup Wizard API