refactor(calendar): group calendars by accounts in settings

Calendars in the "Select Calendars" settings section are now grouped by their source account with color indicators, making it possible to distinguish calendars that share the same name across different accounts.

Each account group header includes a toggle to select or deselect all calendars in that group.

DynamicIsland/managers/CalendarManager.swift

@@ -382,6 +382,36 @@ class CalendarManager: ObservableObject {
         lastEventsFetchDate = Date()
     }
 
+    func setCalendarsSelected(_ calendars: [CalendarModel], isSelected: Bool) async {
+        var selectionState = Defaults[.calendarSelectionState]
+        let ids = Set(calendars.map { $0.id })
+
+        switch selectionState {
+        case .all:
+            if !isSelected {
+                let identifiers = Set(allCalendars.map { $0.id }).subtracting(ids)
+                selectionState = .selected(identifiers)
+            }
+        case .selected(var identifiers):
+            if isSelected {
+                identifiers.formUnion(ids)
+            } else {
+                identifiers.subtract(ids)
+            }
+
+            if identifiers.isEmpty || identifiers.count == allCalendars.count {
+                selectionState = .all
+            } else {
+                selectionState = .selected(identifiers)
+            }
+        }
+
+        Defaults[.calendarSelectionState] = selectionState
+        updateSelectedCalendars()
+        await updateEvents(force: true)
+        await updateLockScreenEvents(force: true)
+    }
+
     func setReminderCompleted(reminderID: String, completed: Bool) async {
         await calendarService.setReminderCompleted(reminderID: reminderID, completed: completed)
         await updateEvents(force: true)

DynamicIsland/managers/LockScreenWidgetPreviewManager.swift

@@ -302,6 +302,7 @@ final class LockScreenWidgetPreviewManager: ObservableObject {
     private func applyCalendarPreviewEvents() {
         let now = Date()
         let reminderCalendar = CalendarModel(
+            accountName: "Preview",
             id: "preview.reminders",
             title: "Reminders",
             color: .systemBlue,
@@ -327,6 +328,7 @@ final class LockScreenWidgetPreviewManager: ObservableObject {
         )
 
         let eventCalendar = CalendarModel(
+            accountName: "Preview",
             id: "preview.calendar",
             title: "Calendar",
             color: .systemOrange,