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/App.tsx

@@ -1,10 +1,11 @@
 import { useState, useRef, useCallback } from 'react';
-import { ConfigProvider, Layout, theme, Typography } from 'antd';
+import { ConfigProvider, Layout, theme, Typography, Space } from 'antd';
+import { ApartmentOutlined } from '@ant-design/icons';
 import { ThemeToggle } from './components/ThemeToggle';
 import { InputPanel } from './components/InputPanel';
 import { MindmapPanel } from './components/MindmapPanel';
 import { useAttribute } from './hooks/useAttribute';
-import type { MindmapD3Ref } from './components/MindmapD3';
+import type { MindmapDAGRef } from './components/MindmapDAG';
 import type { CategoryMode } from './types';
 
 const { Header, Sider, Content } = Layout;
@@ -22,7 +23,7 @@ function App() {
     nodeSpacing: 32,
     fontSize: 14,
   });
-  const mindmapRef = useRef<MindmapD3Ref>(null);
+  const mindmapRef = useRef<MindmapDAGRef>(null);
 
   const handleAnalyze = async (
     query: string,
@@ -36,12 +37,8 @@ function App() {
     await analyze(query, model, temperature, chainCount, categoryMode, customCategories, suggestedCategoryCount);
   };
 
-  const handleExpandAll = useCallback(() => {
-    mindmapRef.current?.expandAll();
-  }, []);
-
-  const handleCollapseAll = useCallback(() => {
-    mindmapRef.current?.collapseAll();
+  const handleResetView = useCallback(() => {
+    mindmapRef.current?.resetView();
   }, []);
 
   return (
@@ -57,11 +54,37 @@ function App() {
             alignItems: 'center',
             justifyContent: 'space-between',
             padding: '0 24px',
+            background: isDark
+              ? 'linear-gradient(90deg, #141414 0%, #1f1f1f 50%, #141414 100%)'
+              : 'linear-gradient(90deg, #fff 0%, #fafafa 50%, #fff 100%)',
+            borderBottom: isDark ? '1px solid #303030' : '1px solid #f0f0f0',
+            boxShadow: isDark
+              ? '0 2px 8px rgba(0, 0, 0, 0.3)'
+              : '0 2px 8px rgba(0, 0, 0, 0.06)',
           }}
         >
-          <Title level={4} style={{ margin: 0, color: isDark ? '#fff' : '#000' }}>
-            Attribute Agent
-          </Title>
+          <Space align="center" size="middle">
+            <ApartmentOutlined
+              style={{
+                fontSize: 24,
+                color: isDark ? '#177ddc' : '#1890ff',
+              }}
+            />
+            <Title
+              level={4}
+              style={{
+                margin: 0,
+                background: isDark
+                  ? 'linear-gradient(90deg, #177ddc 0%, #69c0ff 100%)'
+                  : 'linear-gradient(90deg, #1890ff 0%, #40a9ff 100%)',
+                WebkitBackgroundClip: 'text',
+                WebkitTextFillColor: 'transparent',
+                backgroundClip: 'text',
+              }}
+            >
+              Attribute Agent
+            </Title>
+          </Space>
           <ThemeToggle isDark={isDark} onToggle={setIsDark} />
         </Header>
         <Layout>
@@ -96,8 +119,7 @@ function App() {
               currentResult={currentResult}
               onAnalyze={handleAnalyze}
               onLoadHistory={loadFromHistory}
-              onExpandAll={handleExpandAll}
-              onCollapseAll={handleCollapseAll}
+              onResetView={handleResetView}
               visualSettings={visualSettings}
               onVisualSettingsChange={setVisualSettings}
             />