feat: add frontend UI for SimpleNote Web

- Vanilla JS frontend with dark theme
- Dashboard with sidebar (libraries tree, tags), document grid, search
- Document viewer with markdown rendering and metadata panel
- Document editor with split write/preview and formatting toolbar
- Login screen with token authentication
- All styled according to UI/UX specs (dark theme, accent #00d4aa)
- API client for all endpoints
- Responsive design

src/index.js

@@ -5,6 +5,8 @@
 
 import express from 'express';
 import cors from 'cors';
+import { join, dirname } from 'path';
+import { fileURLToPath } from 'url';
 import config from './config/index.js';
 import { createApiRouter } from './routes/index.js';
 import { errorHandler } from './middleware/errorHandler.js';
@@ -13,6 +15,9 @@ import { ensureDir } from './utils/fsHelper.js';
 
 const app = express();
 
+// Serve static files from public/
+app.use(express.static(join(dirname(fileURLToPath(import.meta.url)), '..', 'public')));
+
 // Middleware
 app.use(cors({ origin: config.corsOrigin }));
 app.use(express.json({ limit: '10mb' }));