[Frontend] Implement `lib/env.ts`

[Muhammadcodes112]

Description

A single typed place to read NEXT_PUBLIC_* environment variables, instead of scattering process.env.X (and its possible-undefined-ness) throughout the codebase.

Where

astre_frontend/lib/env.ts (new file)

What to implement

Export a typed object or set of functions for reading NEXT_PUBLIC_API_BASE_URL and NEXT_PUBLIC_STELLAR_NETWORK. For NEXT_PUBLIC_API_BASE_URL, throw a clear error at call time if it's unset (fail loud, not with a silent wrong default) — see lib/contract-service.ts's existing NEXT_PUBLIC_CONTRACT_ID check for the pattern to follow.

Acceptance Criteria

• Reading a set env var returns its value
• Reading NEXT_PUBLIC_API_BASE_URL when unset throws a clear, specific error message (not undefined silently flowing through)
• npm run typecheck passes