feat: add last message preview to session picker

Read the final user/assistant message from session transcripts and display
it in the picker alongside the session update time. Allows quick previews
of what's in each session without opening it.

src/gateway/session-utils.types.ts

@@ -14,6 +14,7 @@ export type GatewaySessionRow = {
   label?: string;
   displayName?: string;
   derivedTitle?: string;
+  lastMessagePreview?: string;
   channel?: string;
   subject?: string;
   groupChannel?: string;