feat: Added zod env vars

[jmderby]

Description

Introduces environment variable validation using Zod in env.ts. Validates keys like REACT_APP_CROSSMINT_API_KEY_PROD ensuring they meet criteria (e.g., non-empty strings). Errors guide setup without halting execution.

Test plan

1. Validation: Misconfigure .env to trigger error messages.
2. Type Safety: Access env properties in TypeScript, expecting IntelliSense to reflect Zod schema types.

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Latest commit: e2ffbbfe9407dbdf7ba07915a7171b989650b8ce

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