DX: Add startup validation for required Environment Variables

[TusharW4ni]

Context / Problem

The template relies on environment variables (like BETTER_AUTH_SECRET, DATABASE_URL). If a new developer forgets to copy .env.example to .env, or misses a key, the app might start but fail later with a confusing runtime error deep within the authentication logic.

Proposed Solution

Validate required environment variables immediately upon startup. If a required variable is missing, throw a clear, human-readable error, causing pnpm dev to fail fast.

Implementation Details

1. You can use Zod in a Nuxt Server Plugin or utilize a community module like @t3-oss/env-nuxt.
2. Example using Zod in a startup hook or module:

   import { z } from 'zod';
   
   const envSchema = z.object({
     DATABASE_URL: z.string().url(),
     BETTER_AUTH_SECRET: z.string().min(1),
     BETTER_AUTH_URL: z.string().url(),
     // ...
   });
   
   envSchema.parse(process.env); // Will throw a clear error if invalid

Acceptance Criteria

• Starting the app without required .env variables fails immediately with a descriptive error message.