Refactor Workflow and Chat Architecture (#68)

* refactor: overhaul workflow and chat architecture

- Separate Chat and Workflow API endpoints and database models
- Use JSONB to store workflow execution context in Postgres
- Convert workflow engine to use pydantic-ai execution graphs inside a Ray task
- Update frontend React components to support standalone workflow creation
- Remove obsolete and broken workflow runner tests

Co-authored-by: zhaoxi826 <198742034+zhaoxi826@users.noreply.github.com>

* refactor: overhaul workflow and chat architecture

- Separate Chat and Workflow API endpoints and database models
- Use JSONB to store workflow execution context in Postgres
- Convert workflow engine to use pydantic-ai execution graphs inside a Ray task
- Update frontend React components to support standalone workflow creation
- Remove obsolete and broken workflow runner tests

Co-authored-by: zhaoxi826 <198742034+zhaoxi826@users.noreply.github.com>

* refactor: overhaul workflow and chat architecture

- Separate Chat and Workflow API endpoints and database models
- Use JSONB to store workflow execution context in Postgres
- Convert workflow engine to use pydantic-ai execution graphs inside a Ray task
- Update frontend React components to support standalone workflow creation
- Move workflow_engine inside workflow package to keep core root clean
- Remove obsolete and broken workflow runner tests

Co-authored-by: zhaoxi826 <198742034+zhaoxi826@users.noreply.github.com>

---------

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>
Co-authored-by: zhaoxi826 <198742034+zhaoxi826@users.noreply.github.com>

frontend/src/components/Chat/LeftPanel.tsx

@@ -98,15 +98,19 @@ export function LeftPanel({
            <span className="text-sm font-semibold text-slate-600 uppercase tracking-wider">
              {activeTab === 'chats' ? 'Chat History' : 'Workflows'}
            </span>
-           {activeTab === 'chats' && (
-             <button
-               onClick={handleNewChat}
-               className="p-1.5 bg-blue-100 text-blue-600 rounded hover:bg-blue-200 transition-colors"
-               title="New Chat"
-             >
-               <Plus size={16} />
-             </button>
-           )}
+           <button
+             onClick={() => {
+               if (activeTab === 'chats') {
+                 handleNewChat();
+               } else {
+                 setSelectedWorkflow('new'); // 设置为一个特殊值，表示进入新建工作流向导
+               }
+             }}
+             className="p-1.5 bg-blue-100 text-blue-600 rounded hover:bg-blue-200 transition-colors"
+             title={activeTab === 'chats' ? 'New Chat' : 'New Workflow'}
+           >
+             <Plus size={16} />
+           </button>
         </div>
 
         <div className="flex-1 p-3 overflow-y-auto">
@@ -115,7 +119,7 @@ export function LeftPanel({
               {loadingWorkflows ? (
                 <div className="text-center text-slate-400 text-sm py-4">Loading workflows...</div>
               ) : workflows.length === 0 ? (
-                <div className="text-center text-slate-400 text-sm py-4">暂无工作流</div>
+                <div className="text-center text-slate-400 text-sm py-4">暂无工作流<br/>点击右上角 + 创建</div>
               ) : (
                 workflows.map((wf) => (
                   <div