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/ThisWeekWidgetView.swift

@@ -89,11 +89,11 @@ struct ThisWeekWidgetView: View {
                 .frame(width: 16, height: 16)
                 .background(isToday ? primary : Color.clear)
                 .clipShape(Circle())
-            ForEach(evs.prefix(6)) { ev in
+            ForEach(evs.prefix(8)) { ev in
                 eventPill(ev)
             }
-            if evs.count > 6 {
-                Text("+\(evs.count - 6)")
+            if evs.count > 8 {
+                Text("+\(evs.count - 8)")
                     .font(.system(size: 6.5))
                     .foregroundStyle(accent)
             }