feat(widget): calendar filter config + fix group-view data leak

Bug fix: publishWidgetSnapshot() now guards against activeGroup != nil,
so group view events/colors never contaminate the widget cache.

Feature: widgets can now be configured via long-press → Edit Widget.
- WidgetCalendar struct + writeCalendars/readCalendars in WidgetData.swift
- calendarKey added to WidgetEvent (backward-compatible decoder)
- CalendarIntent.swift: CalendarAppEntity + CalendarEntityQuery + CalendarSelectionIntent
- CalendarrTimelineProvider migrated from TimelineProvider to AppIntentTimelineProvider
- All 13 StaticConfiguration widgets changed to AppIntentConfiguration
- publishWidgetSnapshot() builds + writes the calendar list for the intent

An empty selection (default) shows all calendars; selecting specific
calendars filters the widget events accordingly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CalendarrWidgets/CalendarIntent.swift

@@ -0,0 +1,42 @@
+import AppIntents
+import WidgetKit
+
+/// An individual calendar option shown in the widget configuration picker.
+struct CalendarAppEntity: AppEntity, Identifiable {
+    let id: String
+    let name: String
+    let colorHex: String
+
+    static var typeDisplayRepresentation: TypeDisplayRepresentation = "Kalender"
+    static var defaultQuery = CalendarEntityQuery()
+
+    var displayRepresentation: DisplayRepresentation {
+        DisplayRepresentation(title: LocalizedStringResource(stringLiteral: name))
+    }
+}
+
+/// Reads available calendars from the App Group container so the widget
+/// extension can offer options without a network call.
+struct CalendarEntityQuery: EntityQuery {
+    func entities(for identifiers: [String]) async throws -> [CalendarAppEntity] {
+        let ids = Set(identifiers)
+        return WidgetStore.readCalendars()
+            .filter { ids.contains($0.id) }
+            .map { CalendarAppEntity(id: $0.id, name: $0.name, colorHex: $0.colorHex) }
+    }
+
+    func suggestedEntities() async throws -> [CalendarAppEntity] {
+        WidgetStore.readCalendars()
+            .map { CalendarAppEntity(id: $0.id, name: $0.name, colorHex: $0.colorHex) }
+    }
+}
+
+/// Widget configuration intent: lets the user pick which calendars to show.
+/// An empty selection means "show all calendars" (the default).
+struct CalendarSelectionIntent: WidgetConfigurationIntent {
+    static var title: LocalizedStringResource = "Kalender auswählen"
+    static var description = IntentDescription("Wähle welche Kalender im Widget angezeigt werden. Leer = alle Kalender.")
+
+    @Parameter(title: "Kalender")
+    var selectedCalendars: [CalendarAppEntity]
+}