feat: Services UI — nested nav, per-service pages, settings migration

Rename Store → Services: ServicesIndex.jsx shows built-in core services
(Email, Calendar, Files) with Manage links, plus the existing add-on
store below.

New service sub-pages at /services/email|calendar|files serve both
admin and peer roles. Admins see connection info, service status, users
list, and an inline config form (port/data-dir). Peers see connection
info and their personal credentials fetched from peerAPI.

Navigation restructured: a Services parent item expands to show the
three sub-pages via a collapsible sidebar group (ChevronDown toggle).
Both admin and peer navigation include the Services group. Sidebar
extracted NavItem/NavList components to eliminate the duplicate mobile/
desktop rendering.

Settings.jsx drops EmailForm, CalendarForm, FilesForm and their
SERVICE_DEFS entries. Port conflict detection and per-service validation
logic extracted to utils/serviceConfig.js, shared by Settings and the
new service pages. Service form flushers are registered without cleanup
so the Apply banner saves dirty config even when the user navigates away
from a service page before clicking Apply.

Legacy routes /email, /calendar, /files, /store redirect to their new
canonical paths.

GET /api/config now includes installed_services so the nav can derive
which add-ons are installed without a separate store fetch.

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

api/routes/config.py

@@ -118,6 +118,7 @@ def get_config():
             'vip_webdav':   _ips['vip_webdav'],
         }
         config['service_configs'] = service_configs
+        config['installed_services'] = config_manager.get_installed_services()
         config['domain_mode'] = identity.get('domain_mode', 'lan')
         config['domain_name'] = identity.get('domain_name', '')
         config['effective_domain'] = config_manager.get_effective_domain()