Initial commit: WYSIWYG Markdown Editor - Go backend + React/TypeScript frontend with Tailwind CSS

Backend:
- Cobra CLI with --data-dir, --port, --host flags
- Gin HTTP server with REST API for markdown CRUD operations
- File storage on disk (.md files only)
- Comprehensive logrus logging
- Backend tests with CRUD round-trip verification

Frontend:
- React 18 + TypeScript + Tailwind CSS
- Markdown editor with live GFM preview (react-markdown + remark-gfm)
- File management UI (list, create, open, save, delete)
- Theme switcher with Dark/Light/System modes
- Responsive design
- Frontend tests with vitest

Testing:
- All backend tests pass (go test ./...)
- All frontend tests pass (npm test)

frontend/src/components/ThemeSwitcher.tsx

@@ -0,0 +1,55 @@
+import { ReactNode } from 'react'
+
+interface ThemeSwitcherProps {
+  darkMode: boolean
+  onToggle: () => void
+}
+
+export const ThemeSwitcher: React.FC<ThemeSwitcherProps> = ({
+  darkMode,
+  onToggle,
+}) => {
+  const icon: ReactNode = darkMode ? (
+    <svg
+      xmlns="http://www.w3.org/2000/svg"
+      className="h-6 w-6"
+      fill="none"
+      viewBox="0 0 24 24"
+      stroke="currentColor"
+    >
+      <path
+        strokeLinecap="round"
+        strokeLinejoin="round"
+        strokeWidth={2}
+        d="M12 3v1m0 16v1m9-9h-1M4 12H3m15.364 6.364l-.707-.707M6.343 6.343l-.707-.707m12.728 0l-.707.707M6.343 17.657l-.707.707M16 12a4 4 0 11-8 0 4 4 0 018 0z"
+      />
+    </svg>
+  ) : (
+    <svg
+      xmlns="http://www.w3.org/2000/svg"
+      className="h-6 w-6"
+      fill="none"
+      viewBox="0 0 24 24"
+      stroke="currentColor"
+    >
+      <path
+        strokeLinecap="round"
+        strokeLinejoin="round"
+        strokeWidth={2}
+        d="M20.354 15.354A9 9 0 018 15.354c1.433-2.035 3.49-3.585 5.84-4.238l.092.009a4 4 0 013.477 3.477l.009.092C15.915 7.505 17.465 9.562 19.5 11a9 9 0 01-1.146 3.354z"
+      />
+    </svg>
+  )
+
+  return (
+    <div className="absolute top-4 right-4 z-10">
+      <button
+        onClick={onToggle}
+        className="p-2 rounded-lg bg-gray-300 dark:bg-dark-surface text-gray-700 dark:text-dark-text hover:bg-gray-400 dark:hover:bg-dark-surface transition-colors shadow-md"
+        aria-label="Toggle theme"
+      >
+        {icon}
+      </button>
+    </div>
+  )
+}