feat: implement WYSIWYG markdown editor

Add complete markdown editor with Go backend and React/TypeScript frontend.

Backend:
- Cobra CLI with configurable host, port, data-dir, static-dir flags
- REST API for CRUD operations on markdown files (GET, POST, PUT, DELETE)
- File storage with flat .md structure
- Comprehensive Logrus logging for all operations
- Static asset serving for frontend

Frontend:
- React 18 + TypeScript + Tailwind CSS
- Live markdown editor with GFM preview (react-markdown)
- File management UI (list, create, open, save, delete)
- Theme system (Light/Dark/System) with localStorage persistence
- Responsive design (320px - 1920px+)

Testing:
- 6 backend tests covering CRUD round-trip, validation, error handling
- 19 frontend tests covering API, theme system, and UI components
- All tests passing with single 'make test' command

Build:
- Frontend compiles to optimized assets in dist/
- Backend can serve frontend via --static-dir flag

frontend/src/hooks/useTheme.ts

@@ -0,0 +1,51 @@
+import { useState, useEffect } from 'react';
+import type { Theme } from '../types';
+
+const THEME_KEY = 'markdown-editor-theme';
+
+export function useTheme() {
+  const [theme, setThemeState] = useState<Theme>(() => {
+    const stored = localStorage.getItem(THEME_KEY);
+    if (stored && ['light', 'dark', 'system'].includes(stored)) {
+      return stored as Theme;
+    }
+    return 'system';
+  });
+  const [resolvedTheme, setResolvedTheme] = useState<'light' | 'dark'>('light');
+
+  useEffect(() => {
+    const root = window.document.documentElement;
+
+    const updateTheme = () => {
+      let resolved: 'light' | 'dark';
+      if (theme === 'system') {
+        resolved = window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light';
+      } else {
+        resolved = theme;
+      }
+      resolvedTheme !== resolved && setResolvedTheme(resolved);
+
+      if (resolved === 'dark') {
+        root.classList.add('dark');
+      } else {
+        root.classList.remove('dark');
+      }
+    };
+
+    updateTheme();
+
+    if (theme === 'system') {
+      const mediaQuery = window.matchMedia('(prefers-color-scheme: dark)');
+      const handler = () => updateTheme();
+      mediaQuery.addEventListener('change', handler);
+      return () => mediaQuery.removeEventListener('change', handler);
+    }
+  }, [theme]);
+
+  const setTheme = (newTheme: Theme) => {
+    setThemeState(newTheme);
+    localStorage.setItem(THEME_KEY, newTheme);
+  };
+
+  return { theme, resolvedTheme, setTheme };
+}