feat: Add dynamic category system for attribute analysis

Backend:
- Add CategoryMode enum with 4 modes (fixed_only, fixed_plus_custom, custom_only, dynamic_auto)
- Add Step 0 for LLM category analysis before attribute generation
- Implement dynamic prompts for Step 1/2 that work with N categories
- Add execute_step0(), resolve_final_categories(), assemble_dynamic_attribute_tree()
- Update SSE events to include step0_start, step0_complete, categories_resolved

Frontend:
- Add CategorySelector component with mode selection, custom category input, and category count slider
- Update types with CategoryDefinition, Step0Result, DynamicStep1Result, DynamicCausalChain
- Update api.ts with new SSE event handlers
- Update useAttribute hook with category parameters
- Integrate CategorySelector into InputPanel
- Fix mindmap to dynamically extract and display N categories (was hardcoded to 4)
- Add CSS styles for depth 5-8 to support more category levels

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

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

backend/app/models/schemas.py

@@ -1,5 +1,6 @@
 from pydantic import BaseModel
-from typing import Optional, List
+from typing import Optional, List, Dict
+from enum import Enum
 
 
 class AttributeNode(BaseModel):
@@ -46,12 +47,17 @@ class CausalChain(BaseModel):
 
 
 class StreamAnalyzeRequest(BaseModel):
-    """多步驟分析請求"""
+    """多步驟分析請求（更新為支持動態類別）"""
     query: str
     model: Optional[str] = None
     temperature: Optional[float] = 0.7
     chain_count: int = 5  # 用戶可設定要生成多少條因果鏈
 
+    # 新增：動態類別支持
+    category_mode: Optional[str] = "dynamic_auto"  # CategoryMode enum 值
+    custom_categories: Optional[List[str]] = None
+    suggested_category_count: int = 3  # 建議 LLM 生成的類別數量
+
 
 class StreamAnalyzeResponse(BaseModel):
     """最終完整結果"""
@@ -59,3 +65,36 @@ class StreamAnalyzeResponse(BaseModel):
     step1_result: Step1Result
     causal_chains: List[CausalChain]
     attributes: AttributeNode
+
+
+# ===== Dynamic category system schemas =====
+
+class CategoryMode(str, Enum):
+    """類別模式"""
+    FIXED_ONLY = "fixed_only"
+    FIXED_PLUS_CUSTOM = "fixed_plus_custom"
+    CUSTOM_ONLY = "custom_only"
+    DYNAMIC_AUTO = "dynamic_auto"
+
+
+class CategoryDefinition(BaseModel):
+    """類別定義"""
+    name: str
+    description: Optional[str] = None
+    is_fixed: bool = True  # LLM 生成的為 False
+    order: int = 0
+
+
+class Step0Result(BaseModel):
+    """Step 0: LLM 分析建議類別"""
+    categories: List[CategoryDefinition]
+
+
+class DynamicStep1Result(BaseModel):
+    """動態版本的 Step 1 結果"""
+    attributes: Dict[str, List[str]]  # {類別名: [屬性列表]}
+
+
+class DynamicCausalChain(BaseModel):
+    """動態版本的因果鏈"""
+    chain: Dict[str, str]  # {類別名: 選中屬性}