feat: add build-time environment variable validation

[Hallab7]

Implement strict validation for all environment variables using zod and @t3-oss/env-nextjs to prevent runtime failures from missing or malformed configuration.

• Add zod and @t3-oss/env-nextjs dependencies
• Create src/env.mjs with validation schema for all NEXT_PUBLIC_ variables
• Validate SOROBAN_RPC_URL and HORIZON_URL as proper URLs
• Validate contract IDs with regex (must start with 'C' and be 56 chars)
• Validate STELLAR_NETWORK as enum (testnet/mainnet/futurenet)
• Import env.mjs in next.config.mjs to enforce validation during build and dev
• Build now fails immediately with clear errors for invalid/missing variables

closes #61

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