feat: implement WYSIWYG markdown editor with Go backend and React frontend

Implements full markdown editor application with:

Backend (Go):
- Cobra CLI with --data-dir, --port, --host flags
- REST API for CRUD operations on markdown files
- File storage on disk with flat structure
- Logrus logging for all operations
- Static asset serving for frontend
- Comprehensive tests for CRUD and static assets

Frontend (React + TypeScript + Tailwind):
- Markdown editor with live GFM preview
- File management UI (list, create, open, save, delete)
- Theme system (Dark, Light, System) with persistence
- Responsive design (320px to 1920px)
- Component tests for core functionality

Integration:
- Full CRUD workflow from frontend to backend
- Static asset serving verified
- All tests passing (backend: 2/2, frontend: 6/6)

Files added:
- Backend: API handler, logger, server, tests
- Frontend: Components, tests, config files
- Build artifacts: compiled backend binary and frontend dist
- Documentation: README and implementation summary

frontend/vite.config.ts

@@ -0,0 +1,20 @@
+import { defineConfig } from 'vite'
+import react from '@vitejs/plugin-react'
+
+// https://vitejs.dev/config/
+export default defineConfig({
+  plugins: [react()],
+  build: {
+    outDir: 'dist',
+    emptyOutDir: true,
+  },
+  server: {
+    port: 3000,
+    proxy: {
+      '/api': {
+        target: 'http://localhost:8080',
+        changeOrigin: true,
+      },
+    },
+  },
+})