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>

backend/app/models/schemas.py

@@ -73,6 +73,7 @@ class CategoryMode(str, Enum):
     """類別模式"""
     FIXED_ONLY = "fixed_only"
     FIXED_PLUS_CUSTOM = "fixed_plus_custom"
+    FIXED_PLUS_DYNAMIC = "fixed_plus_dynamic"  # Fixed + LLM suggested
     CUSTOM_ONLY = "custom_only"
     DYNAMIC_AUTO = "dynamic_auto"
 
@@ -98,3 +99,35 @@ class DynamicStep1Result(BaseModel):
 class DynamicCausalChain(BaseModel):
     """動態版本的因果鏈"""
     chain: Dict[str, str]  # {類別名: 選中屬性}
+
+
+# ===== DAG (Directed Acyclic Graph) schemas =====
+
+class DAGNode(BaseModel):
+    """DAG 節點 - 每個屬性只出現一次"""
+    id: str                    # 唯一 ID: "{category}_{index}"
+    name: str                  # 顯示名稱
+    category: str              # 所屬類別
+    order: int                 # 欄位內位置
+
+
+class DAGEdge(BaseModel):
+    """DAG 邊 - 節點之間的連接"""
+    source_id: str
+    target_id: str
+
+
+class AttributeDAG(BaseModel):
+    """完整 DAG 結構"""
+    query: str
+    categories: List[CategoryDefinition]
+    nodes: List[DAGNode]
+    edges: List[DAGEdge]
+
+
+class DAGRelationship(BaseModel):
+    """Step 2 輸出 - 單一關係"""
+    source_category: str
+    source: str  # source attribute name
+    target_category: str
+    target: str  # target attribute name