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/css/app.css

@@ -2002,12 +2002,21 @@ a { color: var(--primary); text-decoration: none; }
   border-radius: 10px;
 }
 .share-user-item {
+  display: flex; align-items: center; gap: 10px;
   padding: 10px 14px;
   cursor: pointer;
   border-bottom: 1px solid var(--border);
 }
 .share-user-item:last-child { border-bottom: none; }
 .share-user-item:hover { background: var(--bg-surface); }
+.share-user-item input[type=checkbox] { flex-shrink: 0; }
+/* "Shared with me" section header row inside the calendar-management table */
+.ct-section-row td {
+  padding-top: 14px; font-size: 12px; font-weight: 600;
+  color: var(--text-3); text-transform: uppercase; letter-spacing: .04em;
+}
+.checkbox-row { display: flex; align-items: center; gap: 8px; cursor: pointer; }
+.checkbox-row input[type=checkbox] { flex-shrink: 0; }
 /* .popup-creator styling moved into the .popup-row / #popup-creator rules above. */
 
 /* ── Groups ─────────────────────────────────────────────────── */