feat: Add expert source selector UI

- Add expert source dropdown in TransformationInputPanel
  - Options: LLM 生成, Wikidata, ConceptNet
  - Shows description for each source
- Pass expertSource through App -> TransformationPanel -> hook -> API
- Default source remains 'llm'

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/src/App.tsx

@@ -10,7 +10,7 @@ import { useAttribute } from './hooks/useAttribute';
 import { getModels } from './services/api';
 import type { MindmapDAGRef } from './components/MindmapDAG';
 import type { TransformationDAGRef } from './components/TransformationDAG';
-import type { CategoryMode } from './types';
+import type { CategoryMode, ExpertSource } from './types';
 
 const { Header, Sider, Content } = Layout;
 const { Title } = Typography;
@@ -44,6 +44,7 @@ function App() {
     custom_experts: undefined,
   });
   const [customExpertsInput, setCustomExpertsInput] = useState('');
+  const [expertSource, setExpertSource] = useState<ExpertSource>('llm');
   const [shouldStartTransform, setShouldStartTransform] = useState(false);
   const [transformLoading, setTransformLoading] = useState(false);
 
@@ -186,6 +187,7 @@ function App() {
                         model={transformModel}
                         temperature={transformTemperature}
                         expertConfig={expertConfig}
+                        expertSource={expertSource}
                         shouldStartTransform={shouldStartTransform}
                         onTransformComplete={() => setShouldStartTransform(false)}
                         onLoadingChange={setTransformLoading}
@@ -226,10 +228,12 @@ function App() {
                 temperature={transformTemperature}
                 expertConfig={expertConfig}
                 customExpertsInput={customExpertsInput}
+                expertSource={expertSource}
                 onModelChange={setTransformModel}
                 onTemperatureChange={setTransformTemperature}
                 onExpertConfigChange={setExpertConfig}
                 onCustomExpertsInputChange={setCustomExpertsInput}
+                onExpertSourceChange={setExpertSource}
                 availableModels={availableModels}
               />
             )}