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/src/api/index.ts

@@ -0,0 +1,51 @@
+import type { FileResponse } from '../types'
+
+const API_BASE = '/api'
+
+async function handleResponse<T>(response: Response): Promise<T> {
+  if (!response.ok) {
+    const error = await response.json().catch(() => ({ error: 'Unknown error' }))
+    throw new Error(error.error || `HTTP ${response.status}`)
+  }
+  if (response.status === 204) {
+    return null as T
+  }
+  return response.json()
+}
+
+export const api = {
+  async listFiles(): Promise<string[]> {
+    const response = await fetch(`${API_BASE}/files`)
+    return handleResponse<string[]>(response)
+  },
+
+  async getFile(name: string): Promise<FileResponse> {
+    const response = await fetch(`${API_BASE}/files/${encodeURIComponent(name)}`)
+    return handleResponse<FileResponse>(response)
+  },
+
+  async createFile(name: string, content: string): Promise<FileResponse> {
+    const response = await fetch(`${API_BASE}/files`, {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json' },
+      body: JSON.stringify({ name, content }),
+    })
+    return handleResponse<FileResponse>(response)
+  },
+
+  async updateFile(name: string, content: string): Promise<FileResponse> {
+    const response = await fetch(`${API_BASE}/files/${encodeURIComponent(name)}`, {
+      method: 'PUT',
+      headers: { 'Content-Type': 'application/json' },
+      body: JSON.stringify({ content }),
+    })
+    return handleResponse<FileResponse>(response)
+  },
+
+  async deleteFile(name: string): Promise<void> {
+    const response = await fetch(`${API_BASE}/files/${encodeURIComponent(name)}`, {
+      method: 'DELETE',
+    })
+    return handleResponse<void>(response)
+  },
+}