feat: implement WYSIWYG markdown editor

Add full-stack markdown editor with Go backend and React frontend.

Backend:
- Cobra CLI with --data-dir, --port, --host flags
- REST API for markdown file CRUD operations
- File storage with flat directory structure
- logrus logging for all operations
- Static file serving for frontend
- Comprehensive tests for CRUD and static assets

Frontend:
- React + TypeScript + Vite + Tailwind CSS
- Live markdown preview with marked (GFM)
- File management: list, create, open, save, delete
- Theme system: Dark/Light/System with persistence
- Responsive design (320px to 1920px)
- Component tests for Editor, Preview, Sidebar

Build:
- Makefile for build, test, and run automation
- Single command testing (make test)

Closes SPEC.md requirements

frontend/package.json

@@ -0,0 +1,43 @@
+{
+  "name": "frontend",
+  "private": true,
+  "version": "0.0.0",
+  "type": "module",
+  "scripts": {
+    "dev": "vite",
+    "build": "tsc -b && vite build",
+    "lint": "eslint .",
+    "preview": "vite preview",
+    "test": "vitest run",
+    "test:watch": "vitest"
+  },
+  "dependencies": {
+    "@types/marked": "^5.0.2",
+    "lucide-react": "^0.563.0",
+    "marked": "^17.0.1",
+    "react": "^19.2.0",
+    "react-dom": "^19.2.0"
+  },
+  "devDependencies": {
+    "@eslint/js": "^9.39.1",
+    "@testing-library/jest-dom": "^6.9.1",
+    "@testing-library/react": "^16.3.2",
+    "@testing-library/user-event": "^14.6.1",
+    "@types/node": "^24.10.1",
+    "@types/react": "^19.2.5",
+    "@types/react-dom": "^19.2.3",
+    "@vitejs/plugin-react": "^5.1.1",
+    "autoprefixer": "^10.4.24",
+    "eslint": "^9.39.1",
+    "eslint-plugin-react-hooks": "^7.0.1",
+    "eslint-plugin-react-refresh": "^0.4.24",
+    "globals": "^16.5.0",
+    "jsdom": "^28.0.0",
+    "postcss": "^8.5.6",
+    "tailwindcss": "^3.4.19",
+    "typescript": "~5.9.3",
+    "typescript-eslint": "^8.46.4",
+    "vite": "^7.2.4",
+    "vitest": "^4.0.18"
+  }
+}