feat: add EditEventScreen with calendar and chat mode support

Add a unified event editor that works in two modes:
- Calendar mode: Create/edit events directly via EventService API
- Chat mode: Edit AI-proposed events before confirming them

The chat mode allows users to modify proposed events (title, time,
recurrence) and persists changes both locally and to the server.

New components: DateTimePicker, ScrollableDropdown, useDropdownPosition
New API: PUT /api/chat/messages/:messageId/proposal

apps/client/src/Themes.tsx

@@ -46,8 +46,8 @@ export const THEMES = {
     messageBorderBg: "#3A3430",
     placeholderBg: "#4A4440",
     calenderBg: "#3D2A1A",
-    confirmButton: "#22c55e",
-    rejectButton: "#ef4444",
+    confirmButton: "#136e34",
+    rejectButton: "#bd1010",
     disabledButton: "#555",
     buttonText: "#FFFFFF",
     textPrimary: "#FFFFFF",