iOS: widget padding, week event limit, long-press day preview

- CalendarDayWidget: add 6pt top padding to header so content isn't flush
  against the widget edge
- ThisWeekWidget: increase per-day event cap from 6 to 8 to prevent "+1"
  overflow when there is vertical space available
- MonthView: add DayContextPreviewView to the long-press context menu using
  .contextMenu(menuItems:preview:); shows all-day events as colored bars
  with chevron continuation arrows, timed events as dot+time+title rows

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

CalendarrWidgets/CalendarDayWidgetView.swift

@@ -40,6 +40,7 @@ struct CalendarDayWidgetView: View {
             let accent  = Color(widgetHex: s.accentColorHex)
             VStack(alignment: .leading, spacing: 0) {
                 header(primary: primary)
+                    .padding(.top, 6)
                 weekStrip(snapshot: s, primary: primary, accent: accent)
                     .padding(.vertical, 3)
                 Rectangle()