feat: non-emoji group icons (SF Symbols) for consistent cross-platform look

Group icons are now semantic keys (people/home/heart/work/school/sports/party/
pet/travel/music/food/star) rendered as SF Symbols in the picker, group list,
switcher, banner and filter — instead of OS emoji that looked different on every
platform. Legacy emoji values still render as a fallback. GroupCombinedView uses
the server display_title.

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

Calendarr iOS/Views/CalendarFilterSheet.swift

@@ -179,7 +179,7 @@ struct CalendarFilterSheet: View {
     private var groupFilterList: some View {
         if let g = groupDetail {
             List {
-                Section(header: Text("\(g.icon ?? "👥") \(g.name)")) {
+                Section(header: Label(g.name, systemImage: GroupIcons.symbol(g.icon))) {
                     ForEach(g.members ?? []) { m in
                         groupRow(name: m.displayName ?? "—",
                                  colorHex: m.color ?? "#4285f4",