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>

frontend/src/App.tsx

@@ -5,6 +5,7 @@ import { InputPanel } from './components/InputPanel';
 import { MindmapPanel } from './components/MindmapPanel';
 import { useAttribute } from './hooks/useAttribute';
 import type { MindmapD3Ref } from './components/MindmapD3';
+import type { CategoryMode } from './types';
 
 const { Header, Sider, Content } = Layout;
 const { Title } = Typography;
@@ -27,9 +28,12 @@ function App() {
     query: string,
     model?: string,
     temperature?: number,
-    chainCount?: number
+    chainCount?: number,
+    categoryMode?: CategoryMode,
+    customCategories?: string[],
+    suggestedCategoryCount?: number
   ) => {
-    await analyze(query, model, temperature, chainCount);
+    await analyze(query, model, temperature, chainCount, categoryMode, customCategories, suggestedCategoryCount);
   };
 
   const handleExpandAll = useCallback(() => {