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/pages/Email.jsx

@@ -1,8 +1,8 @@
 import { useState, useEffect } from 'react';
 import { Mail, Users, Wifi, Copy, CheckCheck } from 'lucide-react';
 import { emailAPI } from '../services/api';
+import { useConfig } from '../contexts/ConfigContext';
 
-const CELL_HOST = 'mail.cell';
 const CELL_IP = '172.20.0.23';
 
 function CopyButton({ text }) {
@@ -32,6 +32,8 @@ function InfoRow({ label, value }) {
 }
 
 function Email() {
+  const { domain = 'cell' } = useConfig();
+  const cellHost = `mail.${domain}`;
   const [users, setUsers] = useState([]);
   const [status, setStatus] = useState(null);
   const [isLoading, setIsLoading] = useState(true);
@@ -78,7 +80,7 @@ function Email() {
             <h3 className="text-lg font-medium text-gray-900">Incoming mail (IMAP)</h3>
           </div>
           <div className="bg-gray-50 rounded-lg px-4 py-2">
-            <InfoRow label="Server"       value={CELL_HOST} />
+            <InfoRow label="Server"       value={cellHost} />
             <InfoRow label="Port"         value={String(status?.imap_port ?? 993)} />
             <InfoRow label="Security"     value="SSL/TLS" />
             <InfoRow label="Direct IP"    value={CELL_IP} />
@@ -92,7 +94,7 @@ function Email() {
             <h3 className="text-lg font-medium text-gray-900">Outgoing mail (SMTP)</h3>
           </div>
           <div className="bg-gray-50 rounded-lg px-4 py-2">
-            <InfoRow label="Server"       value={CELL_HOST} />
+            <InfoRow label="Server"       value={cellHost} />
             <InfoRow label="Port"         value={String(status?.smtp_port ?? 587)} />
             <InfoRow label="Security"     value="STARTTLS" />
             <InfoRow label="Auth"         value="Username + Password" />
@@ -106,8 +108,8 @@ function Email() {
             <h3 className="text-lg font-medium text-gray-900">Webmail</h3>
           </div>
           <div className="bg-gray-50 rounded-lg px-4 py-2">
-            <InfoRow label="URL"          value="http://mail.cell" />
-            <InfoRow label="Alt URL"      value="http://webmail.cell" />
+            <InfoRow label="URL"          value={`http://mail.${domain}`} />
+            <InfoRow label="Alt URL"      value={`http://webmail.${domain}`} />
             <InfoRow label="Direct IP"    value={`http://${CELL_IP}`} />
           </div>
           <p className="text-xs text-gray-400 mt-3">