feat: relative timestamps, message preview, and recency sort in sidebar

- Show relative time (2m, 3h, 1d) next to each session name
- Display last message preview below session name (truncated to 80 chars)
- Sort sessions by most recently updated (within pinned/unpinned groups)
- Map updatedAt and lastMessagePreview from gateway sessions.list response

src/types/index.ts

@@ -30,6 +30,8 @@ export interface Session {
   kind?: string;
   model?: string;
   agentId?: string;
+  updatedAt?: number;
+  lastMessagePreview?: string;
 }
 
 export type ConnectionStatus = 'disconnected' | 'connecting' | 'connected';