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/contexts/ConfigContext.jsx

@@ -0,0 +1,22 @@
+import { createContext, useContext, useState, useEffect, useCallback } from 'react';
+import { cellAPI } from '../services/api';
+
+const ConfigContext = createContext({ domain: 'cell', cell_name: 'mycell' });
+
+export function ConfigProvider({ children }) {
+  const [config, setConfig] = useState({ domain: 'cell', cell_name: 'mycell' });
+
+  const refresh = useCallback(() => {
+    cellAPI.getConfig().then(r => setConfig(r.data)).catch(() => {});
+  }, []);
+
+  useEffect(() => { refresh(); }, [refresh]);
+
+  return (
+    <ConfigContext.Provider value={{ ...config, refresh }}>
+      {children}
+    </ConfigContext.Provider>
+  );
+}
+
+export const useConfig = () => useContext(ConfigContext);