feat: Migrate to React Flow and add Fixed + Dynamic category mode

Frontend:
- Migrate MindmapDAG from D3.js to React Flow (@xyflow/react)
- Add custom node components (QueryNode, CategoryHeaderNode, AttributeNode)
- Add useDAGLayout hook for column-based layout
- Add "AI" badge for LLM-suggested categories
- Update CategorySelector with Fixed + Dynamic mode option
- Improve dark/light theme support

Backend:
- Add FIXED_PLUS_DYNAMIC category mode
- Filter duplicate category names in LLM suggestions
- Update prompts to exclude fixed categories when suggesting new ones
- Improve LLM service with better error handling and logging
- Auto-remove /no_think prefix for non-Qwen models
- Add smart JSON format detection for model compatibility
- Improve JSON extraction with multiple parsing strategies

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

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

frontend/src/services/api.ts

@@ -1,13 +1,11 @@
 import type {
   ModelListResponse,
   StreamAnalyzeRequest,
-  StreamAnalyzeResponse,
   Step1Result,
-  CausalChain,
   Step0Result,
   CategoryDefinition,
   DynamicStep1Result,
-  DynamicCausalChain
+  DAGStreamAnalyzeResponse
 } from '../types';
 
 // 自動使用當前瀏覽器的 hostname，支援遠端存取
@@ -19,10 +17,9 @@ export interface SSECallbacks {
   onCategoriesResolved?: (categories: CategoryDefinition[]) => void;
   onStep1Start?: () => void;
   onStep1Complete?: (result: Step1Result | DynamicStep1Result) => void;
-  onChainStart?: (index: number, total: number) => void;
-  onChainComplete?: (index: number, chain: CausalChain | DynamicCausalChain) => void;
-  onChainError?: (index: number, error: string) => void;
-  onDone?: (response: StreamAnalyzeResponse) => void;
+  onRelationshipsStart?: () => void;
+  onRelationshipsComplete?: (count: number) => void;
+  onDone?: (response: DAGStreamAnalyzeResponse) => void;
   onError?: (error: string) => void;
 }
 
@@ -87,14 +84,11 @@ export async function analyzeAttributesStream(
             case 'step1_complete':
               callbacks.onStep1Complete?.(eventData.result);
               break;
-            case 'chain_start':
-              callbacks.onChainStart?.(eventData.index, eventData.total);
+            case 'relationships_start':
+              callbacks.onRelationshipsStart?.();
               break;
-            case 'chain_complete':
-              callbacks.onChainComplete?.(eventData.index, eventData.chain);
-              break;
-            case 'chain_error':
-              callbacks.onChainError?.(eventData.index, eventData.error);
+            case 'relationships_complete':
+              callbacks.onRelationshipsComplete?.(eventData.count);
               break;
             case 'done':
               callbacks.onDone?.(eventData);