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/Calendar.jsx

@@ -1,8 +1,8 @@
 import { useState, useEffect } from 'react';
 import { Calendar as CalendarIcon, Users, Wifi, Copy, CheckCheck } from 'lucide-react';
 import { calendarAPI } from '../services/api';
+import { useConfig } from '../contexts/ConfigContext';
 
-const CELL_HOST = 'calendar.cell';
 const CELL_IP = '172.20.0.21';
 
 function CopyButton({ text }) {
@@ -32,6 +32,8 @@ function InfoRow({ label, value }) {
 }
 
 function Calendar() {
+  const { domain = 'cell' } = useConfig();
+  const cellHost = `calendar.${domain}`;
   const [users, setUsers] = useState([]);
   const [status, setStatus] = useState(null);
   const [isLoading, setIsLoading] = useState(true);
@@ -81,9 +83,9 @@ function Calendar() {
             Use these settings in your calendar / contacts app (iOS, Android, Thunderbird, etc.)
           </p>
           <div className="bg-gray-50 rounded-lg px-4 py-2">
-            <InfoRow label="Server URL"    value={`http://${CELL_HOST}`} />
-            <InfoRow label="CalDAV path"   value={`http://${CELL_HOST}/`} />
-            <InfoRow label="CardDAV path"  value={`http://${CELL_HOST}/`} />
+            <InfoRow label="Server URL"    value={`http://${cellHost}`} />
+            <InfoRow label="CalDAV path"   value={`http://${cellHost}/`} />
+            <InfoRow label="CardDAV path"  value={`http://${cellHost}/`} />
             <InfoRow label="Port"          value="80" />
             <InfoRow label="Direct IP"     value={CELL_IP} />
             <InfoRow label="Protocol"      value="HTTP (CalDAV/CardDAV)" />
@@ -104,7 +106,7 @@ function Calendar() {
               <p className="font-medium text-gray-900 mb-1">iOS (Settings → Calendar → Accounts)</p>
               <ol className="list-decimal ml-4 space-y-0.5 text-xs text-gray-600">
                 <li>Add Account → Other → Add CalDAV Account</li>
-                <li>Server: <span className="font-mono">calendar.cell</span></li>
+                <li>Server: <span className="font-mono">{cellHost}</span></li>
                 <li>Enter username &amp; password</li>
                 <li>For contacts: Add CardDAV Account, same server</li>
               </ol>
@@ -113,7 +115,7 @@ function Calendar() {
               <p className="font-medium text-gray-900 mb-1">Android (DAVx⁵ app)</p>
               <ol className="list-decimal ml-4 space-y-0.5 text-xs text-gray-600">
                 <li>Install DAVx⁵ from Play Store / F-Droid</li>
-                <li>Login with URL: <span className="font-mono">http://calendar.cell/</span></li>
+                <li>Login with URL: <span className="font-mono">http://{cellHost}/</span></li>
                 <li>Select calendars &amp; address books to sync</li>
               </ol>
             </div>
@@ -121,7 +123,7 @@ function Calendar() {
               <p className="font-medium text-gray-900 mb-1">Thunderbird</p>
               <ol className="list-decimal ml-4 space-y-0.5 text-xs text-gray-600">
                 <li>Calendar → New Calendar → On the Network</li>
-                <li>Format: CalDAV, Location: <span className="font-mono">http://calendar.cell/</span></li>
+                <li>Format: CalDAV, Location: <span className="font-mono">http://{cellHost}/</span></li>
               </ol>
             </div>
           </div>