fix: all service pages use live domain; cell_name/domain propagate to DNS; /api/status reads stored identity

Changes:
- ConfigContext.jsx: React context that loads /api/config once; exposes domain,
  cell_name, refresh() — wraps entire app in App.jsx
- Email/Calendar/Files pages: replace hardcoded 'mail.cell', 'calendar.cell',
  'files.cell', 'webdav.cell' with domain from ConfigContext; hostname updates
  immediately after Settings save (refreshConfig() called on save)
- /api/status: cell_name and domain now read from stored _identity in config_manager,
  not hardcoded 'personal-internet-cell' / 'cell.local'
- network_manager.apply_cell_name(old, new): updates hostname A-record in primary
  zone file and reloads CoreDNS; called from PUT /api/config when cell_name changes
- Old identity captured before save so apply_cell_name gets the correct old value
- Settings EmailForm: smtp/imap ports are read-only with note (docker-compose.yml level)
- Settings FilesForm: port is read-only with note (Caddy proxies on 80 externally)
- Settings CalendarForm: port labeled "Internal port; clients use 80 via Caddy"

Tests added:
- test_apply_cell_name_renames_host_record
- test_apply_cell_name_noop_when_same

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

webui/src/App.jsx

@@ -16,6 +16,7 @@ import {
   Settings as SettingsIcon
 } from 'lucide-react';
 import { healthAPI } from './services/api';
+import { ConfigProvider } from './contexts/ConfigContext';
 import Sidebar from './components/Sidebar';
 import Dashboard from './pages/Dashboard';
 import Peers from './pages/Peers';
@@ -81,6 +82,7 @@ function App() {
 
   return (
     <Router>
+      <ConfigProvider>
       <div className="min-h-screen bg-gray-50">
         <Sidebar navigation={navigation} isOnline={isOnline} />
         
@@ -126,6 +128,7 @@ function App() {
           </main>
         </div>
       </div>
+      </ConfigProvider>
     </Router>
   );
 }