FN-5884: preserve quick chat session across reopens

Keep quick chat warm and restore the last opened conversation per project.

- persist the active quick chat session id in per-project local storage and restore it before falling back to latest activity
- retain in-memory quick chat state across close/reopen while resetting chat state correctly when the project changes
- add storage, hook, and FAB coverage for restored sessions, warm reopen behavior, and project switching; document the updated quick chat behavior

Files changed:
 docs/dashboard-guide.md                            |   3 +-
 packages/dashboard/app/components/QuickChatFAB.tsx |  29 ++++--
 .../app/components/__tests__/QuickChatFAB.test.tsx | 112 +++++++++++++++++++--
 .../__tests__/quickChatLastSessionStorage.test.ts  |  55 ++++++++++
 .../app/hooks/__tests__/useQuickChat.test.ts       |  70 +++++++++++++
 .../app/hooks/quickChatLastSessionStorage.ts       |  41 ++++++++
 packages/dashboard/app/hooks/useQuickChat.ts       |  25 +++++
 7 files changed, 319 insertions(+), 16 deletions(-)

Fusion-Task-Id: FN-5884

Fusion-Task-Lineage: 4d689611-1824-41db-8fc2-d543407316ff

packages/dashboard/app/hooks/useQuickChat.ts

@@ -26,6 +26,7 @@ import {
   removePersistedPendingChatMessage,
   setPersistedPendingChatMessage,
 } from "./chatPendingMessageStorage";
+import { setPersistedLastQuickChatSessionId } from "./quickChatLastSessionStorage";
 import { isLikelyTabSuspensionError, useTabVisibilitySuspension } from "./visibilitySuspension";
 
 interface ModelSelection {
@@ -251,6 +252,14 @@ export function useQuickChat(
     lastAttachedGenerationRef.current = null;
   }, [projectId]);
 
+  useEffect(() => {
+    if (!activeSession?.id) {
+      return;
+    }
+
+    setPersistedLastQuickChatSessionId(projectId, activeSession.id);
+  }, [activeSession?.id, projectId]);
+
   const refreshSessions = useCallback(async () => {
     setSessionsLoading(true);
     try {
@@ -532,6 +541,22 @@ export function useQuickChat(
     setIsStreaming(false);
   }, []);
 
+  useEffect(() => {
+    if (streamRef.current) {
+      streamRef.current.close();
+      streamRef.current = null;
+    }
+
+    lastAttachedGenerationRef.current = null;
+    currentSessionKeyRef.current = "";
+    currentSessionTargetRef.current = null;
+    activeSessionRef.current = null;
+    setActiveSession(null);
+    setMessages([]);
+    setSessions([]);
+    resetTransientComposerState();
+  }, [projectId, resetTransientComposerState]);
+
   // Switch to a different chat target session
   const switchSession = useCallback(
     async (agentId: string, modelProvider?: string, modelId?: string) => {