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/main.py

@@ -244,6 +244,14 @@ def _migrate():
         except Exception:
             pass
 
+        # Hide a user from sharing/group pickers (admin management still shows them).
+        try:
+            conn.execute(text("ALTER TABLE users ADD COLUMN directory_hidden BOOLEAN DEFAULT 0"))
+            conn.commit()
+            logging.info("Migration: added directory_hidden to users")
+        except Exception:
+            pass
+
 _migrate()
 
 app = FastAPI(title="Calendarr", docs_url=None, redoc_url=None)