fix: Liquid Glass month title updates via @Observable store (visibleMonth)

The system NavigationStack toolbar title would not refresh on a plain @State
change (title kept disappearing on iPhone). Moved visibleMonth into the
@Observable CalendarStore so the toolbar's read is tracked with @Observable's
fine-grained observation and refreshes on month change. Reverted the @State/.id
workaround. Real system glass bar retained.

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

Calendarr iOS/Models/CalendarStore.swift

@@ -51,6 +51,10 @@ class CalendarStore {
     var events: [CalEvent] = []
     var viewType: CalViewType = .month
     var currentDate: Date = .now
+    // The month currently scrolled into view (month view). Lives in the store so
+    // the Liquid-Glass navigation title — read in the system toolbar — updates
+    // via @Observable tracking (a plain @State did not refresh the toolbar).
+    var visibleMonth: Date = .now
     var isLoading = false
     var isCachingBackground = false
     var lastError: String? = nil