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/hooks/useExpertTransformation.ts

@@ -7,11 +7,13 @@ import type {
   ExpertTransformationDAGResult,
   ExpertProfile,
   CategoryDefinition,
+  ExpertSource,
 } from '../types';
 
 interface UseExpertTransformationOptions {
   model?: string;
   temperature?: number;
+  expertSource?: ExpertSource;
 }
 
 export function useExpertTransformation(options: UseExpertTransformationOptions = {}) {
@@ -60,6 +62,7 @@ export function useExpertTransformation(options: UseExpertTransformationOptions
             expert_count: expertConfig.expert_count,
             keywords_per_expert: expertConfig.keywords_per_expert,
             custom_experts: expertConfig.custom_experts,
+            expert_source: options.expertSource,
             model: options.model,
             temperature: options.temperature,
           },