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/tailwind.config.cjs

@@ -0,0 +1,12 @@
+/** @type {import('tailwindcss').Config} */
+module.exports = {
+  content: [
+    "./index.html",
+    "./src/**/*.{js,ts,jsx,tsx}",
+  ],
+  darkMode: 'class',
+  theme: {
+    extend: {},
+  },
+  plugins: [],
+}