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/components/ModalBase.tsx

@@ -9,6 +9,7 @@ type ModalBaseProps = {
   title: string;
   subtitle?: string;
   children: ReactNode;
+  attachment?: ReactNode;
   footer?: {
     label: string;
     onPress: () => void;
@@ -23,6 +24,7 @@ export const ModalBase = ({
   title,
   subtitle,
   children,
+  attachment,
   footer,
   scrollable,
   maxContentHeight,
@@ -55,6 +57,7 @@ export const ModalBase = ({
           <CardBase
             title={title}
             subtitle={subtitle}
+            attachment={attachment}
             footer={footer}
             scrollable={scrollable}
             maxContentHeight={maxContentHeight}