Implement message preview feature for support tickets

- Added a new property `MessagePreview` to the `TicketSummaryDto` to display the truncated first message of a ticket.
- Updated the `TicketMapping` class to retrieve the first message for each ticket and truncate it for preview purposes.
- Modified the `SupportTicketList` and `AdminSupportPage` components to conditionally render the message preview in the ticket list, enhancing user experience by providing context at a glance.

frontend/src/shared/api/types.ts

@@ -267,6 +267,7 @@ export type TicketSummaryDto = {
   status: TicketStatus
   createdAt: string
   lastActivityAt: string
+  messagePreview: string | null
 }
 
 export type TicketDetailDto = {