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/index.html

@@ -742,6 +742,13 @@
               <option value="hidden" data-i18n="private_visibility_hidden">Ausblenden</option>
             </select>
           </div>
+          <div class="form-group">
+            <label class="checkbox-row">
+              <input type="checkbox" id="cfg-directory-hidden" />
+              <span data-i18n="settings_directory_hidden">Profil verbergen (nicht in Teilen-/Gruppen-Auswahl anzeigen)</span>
+            </label>
+            <p class="panel-desc" data-i18n="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.</p>
+          </div>
 
           <h4 class="panel-title" style="margin-top:24px" data-i18n="settings_default_duration">Standard-Termindauer</h4>
           <div class="contrast-selector" id="cfg-event-duration">