[Docs] Add frontend/.env.example documenting NEXT_PUBLIC_* variables

[grantfox-oss]

Telegram (ask questions / claim the issue here first): https://t.me/+DOylgFv1jyJlNzM0

Why this matters

The frontend reads 11 NEXT_PUBLIC_* vars in code (NEXT_PUBLIC_API_URL, NEXT_PUBLIC_APP_VERSION, NEXT_PUBLIC_STELLAR_NETWORK, NEXT_PUBLIC_STREAMING_CONTRACT, NEXT_PUBLIC_STREAM_CONTRACT_ID, NEXT_PUBLIC_SOROBAN_RPC_URL, NEXT_PUBLIC_NETWORK_PASSPHRASE, NEXT_PUBLIC_USDC_ADDRESS, NEXT_PUBLIC_EURC_ADDRESS, NEXT_PUBLIC_XLM_ADDRESS, NEXT_PUBLIC_STELLAR_EXPERT_URL) but there is no frontend/.env.example, and CONTRIBUTING.md only says create a .env.local if needed.

Acceptance criteria

• Add frontend/.env.example listing every NEXT_PUBLIC_* var the code reads, with comments and sensible testnet defaults
• Reference it from frontend setup docs
• Expand the frontend env table in docs/ARCHITECTURE.md to cover the full set

Files to touch

• frontend/.env.example
• docs/ARCHITECTURE.md
• CONTRIBUTING.md

Out of scope

• backend/.env.example