feat: hide individual member calendars in the group view (iOS)

The calendar filter, when a group overlay is active, now lists the group's
members (+ the shared group calendar) and lets you hide each one individually
(Outlook-style). Filtering is client-side via CalendarStore.hiddenGroupKeys
(per-member gm:<id> / group-calendar gc keys), reset when switching groups.

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

Calendarr iOS/Views/Calendar/CalendarHostView.swift

@@ -247,6 +247,7 @@ struct CalendarHostView: View {
 
     private func switchGroup(_ g: CalGroup?) {
         store.activeGroup = g
+        store.hiddenGroupKeys = []   // member visibility is per-group; start fresh
         // The cache holds the previous mode's events — drop it and reload the
         // visible range + prefetch a wide window so the whole grid is covered.
         Task { await forceReload() }