feat(markdown-editor): implement wysiswyg markdown editor with live preview

- Build Go backend with Cobra CLI and REST API
  - CRUD operations for markdown files (GET, POST, PUT, DELETE)
  - File storage with flat .md file structure
  - Comprehensive logrus logging with JSON format
  - Static asset serving for frontend

- Build React/TypeScript frontend with Tailwind CSS
  - Markdown editor with live GFM preview
  - File management UI (list, create, open, delete)
  - Theme system (Dark/Light/System) with persistence
  - Responsive design (320px mobile, 1920px desktop)

- Add comprehensive test coverage
  - Backend: API, storage, and logger tests (13 tests passing)
  - Frontend: Editor and App component tests

- Setup Nix development environment with Go, Node.js, and TypeScript

backend/test-api.sh

@@ -0,0 +1,74 @@
+#!/bin/bash
+
+# Start server in background
+echo "Starting server..."
+cd backend
+./server &
+SERVER_PID=$!
+
+# Wait for server to start
+sleep 2
+
+# Test API endpoints
+echo ""
+echo "Testing API endpoints..."
+echo ""
+
+# Test 1: List files (should be empty initially)
+echo "1. Testing GET /api/files"
+curl -s http://127.0.0.1:8080/api/files
+echo ""
+echo ""
+
+# Test 2: Create a file
+echo "2. Testing POST /api/files"
+curl -s -X POST http://127.0.0.1:8080/api/files \
+  -H "Content-Type: application/json" \
+  -d '{"name":"test.md","content":"# Test Heading\n\nTest content."}'
+echo ""
+echo ""
+
+# Test 3: List files (should have one file)
+echo "3. Testing GET /api/files"
+curl -s http://127.0.0.1:8080/api/files
+echo ""
+echo ""
+
+# Test 4: Get a file
+echo "4. Testing GET /api/files/test.md"
+curl -s http://127.0.0.1:8080/api/files/test.md
+echo ""
+echo ""
+
+# Test 5: Update a file
+echo "5. Testing PUT /api/files/test.md"
+curl -s -X PUT http://127.0.0.1:8080/api/files/test.md \
+  -H "Content-Type: application/json" \
+  -d '{"content":"# Updated Heading\n\nUpdated content."}'
+echo ""
+echo ""
+
+# Test 6: List files (should still have one file)
+echo "6. Testing GET /api/files"
+curl -s http://127.0.0.1:8080/api/files
+echo ""
+echo ""
+
+# Test 7: Delete a file
+echo "7. Testing DELETE /api/files/test.md"
+curl -s -X DELETE http://127.0.0.1:8080/api/files/test.md
+echo ""
+echo ""
+
+# Test 8: List files (should be empty again)
+echo "8. Testing GET /api/files"
+curl -s http://127.0.0.1:8080/api/files
+echo ""
+echo ""
+
+# Cleanup
+echo "Stopping server..."
+kill $SERVER_PID
+wait $SERVER_PID 2>/dev/null
+
+echo "API test completed successfully!"