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>

backend/routers/users_router.py

@@ -53,7 +53,10 @@ def user_directory(
     """
     users = (
         db.query(models.User)
-        .filter(models.User.id != current_user.id)
+        .filter(
+            models.User.id != current_user.id,
+            models.User.directory_hidden == False,  # noqa: E712 — hidden users opt out of pickers
+        )
         .order_by(models.User.username)
         .all()
     )