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

- Backend: Go HTTP server with Cobra CLI (--data-dir, --port, --host flags)
- CRUD REST API for markdown files with JSON error responses
- File storage in flat directory structure (flat structure, .md files only)
- Comprehensive logrus logging for all operations
- Static file serving for frontend build (./frontend/dist)
- Frontend: React + TypeScript + Tailwind CSS
- Markdown editor with live GFM preview
- File management: list, create, open, save, delete
- Theme system (Dark, Light, System) with persistence
- Responsive design for desktop and mobile
- Backend tests (storage, API handlers) and frontend tests

frontend/src/index.css

@@ -0,0 +1,20 @@
+@tailwind base;
+@tailwind components;
+@tailwind utilities;
+
+* {
+  box-sizing: border-box;
+  margin: 0;
+  padding: 0;
+}
+
+body {
+  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, 'Helvetica Neue', Arial, sans-serif;
+  line-height: 1.5;
+  color: #333;
+  background: #fff;
+}
+
+#root {
+  min-height: 100vh;
+}