feat(FN-2306): add fresh quick-chat thread creation flow

- Add startFreshSession to useQuickChat so users can explicitly create a new persisted session for the current agent/model target
- Refactor session creation into a shared helper and track current session target to support fresh-thread creation without changing selection
- Add a New chat action to the QuickChatFAB header that starts a fresh thread while preserving active model/agent context
- Update quick chat hook/component tests to verify new-session behavior and message streaming targets
- Add header action styling for the new quick-chat controls

packages/dashboard/app/hooks/__tests__/useQuickChat.test.ts

@@ -203,6 +203,51 @@ describe("useQuickChat", () => {
     });
   });
 
+  it("startFreshSession creates a second session for the same model target", async () => {
+    const existingSession = makeSession({
+      id: "session-existing",
+      agentId: FN_AGENT_ID,
+      modelProvider: "openai",
+      modelId: "gpt-4o",
+    });
+    const freshSession = makeSession({
+      id: "session-fresh",
+      agentId: FN_AGENT_ID,
+      modelProvider: "openai",
+      modelId: "gpt-4o",
+    });
+
+    mockFetchChatSessions.mockResolvedValueOnce({ sessions: [existingSession] });
+    mockCreateChatSession.mockResolvedValueOnce({ session: freshSession });
+
+    const { result } = renderHook(() => useQuickChat("proj-123"));
+
+    await act(async () => {
+      await result.current.startModelChat("openai", "gpt-4o");
+    });
+
+    await waitFor(() => {
+      expect(result.current.activeSession?.id).toBe("session-existing");
+    });
+
+    await act(async () => {
+      await result.current.startFreshSession();
+    });
+
+    await waitFor(() => {
+      expect(mockCreateChatSession).toHaveBeenCalledWith(
+        {
+          agentId: FN_AGENT_ID,
+          modelProvider: "openai",
+          modelId: "gpt-4o",
+        },
+        "proj-123",
+      );
+      expect(result.current.activeSession?.id).toBe("session-fresh");
+      expect(mockFetchChatMessages).toHaveBeenCalledWith("session-fresh", { limit: 50 }, "proj-123");
+    });
+  });
+
   it("stopStreaming aborts stream and resets streaming state", async () => {
     const existingSession = makeSession({ id: "session-existing", agentId: "agent-001" });
     const closeFn = vi.fn();