feat(sharing): hidden-profile toggle + hide non-sharing group members

- UserProfile decodes directory_hidden; updateProfile() sends it; Settings has
  a "hide my profile" toggle (excludes you from share/group pickers).
- GroupMember decodes shares_calendar; the group filter list now only shows
  members who actually share a calendar (no phantom empty rows).
- Robust custom decoders (decodeIfPresent) for the new fields.

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

Calendarr iOS/Views/CalendarFilterSheet.swift

@@ -217,7 +217,9 @@ struct CalendarFilterSheet: View {
         if let g = groupDetail {
             List {
                 Section(header: Label(g.name, systemImage: GroupIcons.symbol(g.icon))) {
-                    ForEach(g.members ?? []) { m in
+                    // Only members who actually share a calendar into the group —
+                    // avoids phantom empty rows for members who share nothing.
+                    ForEach((g.members ?? []).filter { $0.sharesCalendar }) { m in
                         groupRow(name: m.displayName ?? "—",
                                  colorHex: m.color ?? "#4285f4",
                                  key: CalendarStore.groupMemberKey(m.id))