feat(calendar): add event context menu and fix event persistence

- Add reusable ContextMenu component to shared-ui with icon support
- Create EventContextMenu for calendar with actions:
  - Edit, Duplicate, Change Calendar, Change Color
  - Export to .ics, Delete with confirmation
- Integrate context menu in DayView, WeekView, and AgendaView
- Fix event creation not showing success/error feedback
- Fix events store to properly handle API response format
- Add API logging for debugging event operations

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

packages/shared-ui/src/context-menu/ContextMenu.svelte

@@ -111,7 +111,7 @@
 				>
 					{#if item.icon}
 						<span class="item-icon">
-							{@render item.icon()}
+							<item.icon size={16} />
 						</span>
 					{/if}
 					<span class="item-label">{item.label}</span>