feat(sharing): group-shared calendars in every member's sidebar, person share picker, hidden profiles

- Backend: co-member group_visible calendars now surface in /local/calendars
  (owned=false, shared_by=owner, read-only, group_shared) and in the normal
  /caldav/events merge (via readable_local_calendar_ids), deduped against
  direct shares / group calendars so nothing appears twice.
- Backend: new User.directory_hidden — a user can hide from sharing/group
  pickers (/users/directory), while admin user management (/users/) still lists
  them. Migration + profile GET/PUT.
- Backend: /groups/{id} members carry shares_calendar so clients can drop
  phantom rows for members who share nothing.
- Frontend: reachable "Teilen" button on owned local calendars; share modal is
  now a checkbox multi-select of users (checked = shared). Hidden-profile toggle
  in Settings → Profile. Group member filter only lists members who actually
  share (phantom fix). Calendars shared with me moved to a dedicated read-only
  "shared with me" section in the manage table.
- Tests: group_visible propagation, no-share absence, dedup, directory_hidden.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/js/i18n.js

@@ -91,6 +91,8 @@ const translations = {
     settings_privacy: 'Privatsphäre',
     settings_private_visibility: 'Private Termine für Gruppenmitglieder',
     settings_private_visibility_desc: 'Wie private Termine für andere Gruppenmitglieder erscheinen',
+    settings_directory_hidden: 'Profil verbergen (nicht in Teilen-/Gruppen-Auswahl anzeigen)',
+    settings_directory_hidden_desc: 'Andere Nutzer können dich dann nicht auswählen, um Kalender zu teilen oder dich zu Gruppen hinzuzufügen. In der Admin-Benutzerverwaltung bleibst du sichtbar.',
     private_visibility_busy: 'Als „Beschäftigt“ anzeigen',
     private_visibility_hidden: 'Ausblenden',
     created_by: 'Erstellt von: {name}',
@@ -406,6 +408,8 @@ const translations = {
     settings_privacy: 'Privacy',
     settings_private_visibility: 'Private events for group members',
     settings_private_visibility_desc: 'How your private events appear to other group members',
+    settings_directory_hidden: 'Hide my profile (don\'t show me in share/group pickers)',
+    settings_directory_hidden_desc: 'Other users then can\'t pick you to share calendars with or add you to groups. You remain visible in the admin user management.',
     private_visibility_busy: 'Show as "Busy"',
     private_visibility_hidden: 'Hide completely',
     created_by: 'Created by: {name}',