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/Models/Localization.swift

@@ -164,6 +164,7 @@ private let strings: [String: [String: String]] = [
         "group.name": "Name",
         "group.icon": "Icon",
         "group.members": "Mitglieder",
+        "group.calendar": "Gruppenkalender",
         "group.member_colors": "Farben der Mitglieder",
         "group.delete": "Gruppe löschen",
 
@@ -465,6 +466,7 @@ private let strings: [String: [String: String]] = [
         "group.name": "Name",
         "group.icon": "Icon",
         "group.members": "Members",
+        "group.calendar": "Group calendar",
         "group.member_colors": "Member colours",
         "group.delete": "Delete group",