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/hooks/useGateway.ts

@@ -97,6 +97,8 @@ export function useGateway() {
           kind: s.kind as string | undefined,
           model: s.model as string | undefined,
           agentId: s.agentId as string | undefined,
+          updatedAt: s.updatedAt as number | undefined,
+          lastMessagePreview: s.lastMessagePreview as string | undefined,
         })));
       }
     } catch {