Add Event Calendar feature with event occurrences service integration

Introduced a new Event Calendar component that displays scheduled events using the Heron.MudCalendar. Implemented IEventOccurrenceService to fetch event occurrences, and added mock data for initial testing. Updated navigation menu to include a link to the Event Calendar.

WebApp/Services/IEventOccurrenceService.cs

@@ -0,0 +1,17 @@
+using Core.Entities;
+
+namespace WebApp.Services;
+
+public interface IEventOccurrenceService
+{
+    /// <summary>
+    /// Gets all event occurrences.
+    /// </summary>
+    Task<IEnumerable<EventOccurrence>> GetEventOccurrencesAsync();
+
+    /// <summary>
+    /// Gets event occurrences within the specified date range.
+    /// </summary>
+    Task<IEnumerable<EventOccurrence>> GetEventOccurrencesForDateRangeAsync(DateTime start, DateTime end);
+}
+