Initial commit: Set up Garmin Health Lab project structure

- Initialize monorepo with root workspace configuration
- Set up Express.js backend with TypeScript
- Set up React 18 frontend with TypeScript
- Create database schema with SQLite
- Implement project architecture and documentation
- Add development and deployment guidelines

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

server/src/middleware/errorHandler.ts

@@ -0,0 +1,31 @@
+import { Request, Response, NextFunction } from 'express';
+
+export function errorHandler(
+  err: any,
+  req: Request,
+  res: Response,
+  next: NextFunction
+) {
+  console.error('Error:', err);
+
+  const status = err.status || 500;
+  const message = err.message || 'Internal server error';
+
+  res.status(status).json({
+    error: {
+      status,
+      message,
+      ...(process.env.NODE_ENV === 'development' && { stack: err.stack })
+    }
+  });
+}
+
+export class AppError extends Error {
+  constructor(
+    public status: number,
+    public message: string
+  ) {
+    super(message);
+    this.name = this.constructor.name;
+  }
+}