feat: show response generation time on assistant messages

Track how long each assistant response took to generate and display
it subtly next to the timestamp (e.g. '· 12.3s'). The timing is
measured from the first streaming delta to the final state, and
preserved across the history reload that follows stream completion.

Only visible for messages generated during the current session.

src/hooks/useGateway.ts

@@ -54,6 +54,8 @@ export function useGateway() {
   const [activeSessions, setActiveSessions] = useState<Set<string>>(new Set());
   const [unreadSessions, setUnreadSessions] = useState<Set<string>>(new Set());
   const [agentIdentity, setAgentIdentity] = useState<AgentIdentity | null>(null);
+  /** Map of runId → generation duration (ms), preserved across loadHistory reloads */
+  const generationTimesRef = useRef<Map<string, number>>(new Map());
 
   const handleAgentEvent = useCallback((payload: JsonPayload) => {
     if (payload?.stream !== 'tool') return;
@@ -228,6 +230,18 @@ export function useGateway() {
             merged.push(msg);
           }
         }
+        // Apply stored generation time to the last assistant message if available
+        const genKey = sessionKey + ':latest';
+        const genTime = generationTimesRef.current.get(genKey);
+        if (genTime) {
+          generationTimesRef.current.delete(genKey);
+          for (let i = merged.length - 1; i >= 0; i--) {
+            if (merged[i].role === 'assistant') {
+              merged[i] = { ...merged[i], generationTimeMs: genTime };
+              break;
+            }
+          }
+        }
         setMessages(merged);
       }
     } catch {
@@ -342,10 +356,16 @@ export function useGateway() {
             blocks,
             isStreaming: true,
             runId,
+            streamStartedAt: Date.now(),
           };
           return [...prev, msg];
         });
       } else if (state === 'final') {
+        // Compute generation time from the streaming message before history reload replaces it
+        const lastMsg = messagesRef.current[messagesRef.current.length - 1];
+        if (lastMsg?.role === 'assistant' && lastMsg.streamStartedAt) {
+          generationTimesRef.current.set(activeSessionRef.current + ':latest', Date.now() - lastMsg.streamStartedAt);
+        }
         currentRunIdRef.current = null;
         setIsGenerating(false);
         loadHistory(activeSessionRef.current);