[WillSewell]

Summary

• Add a "Running on Mainnet" section to the react-svm example README covering VITE_PROGRAM_ID and VITE_RPC_URL, which both default to Solana devnet.
• Add a commented VITE_PROGRAM_ID entry to .env.example and note the mainnet overrides alongside VITE_RPC_URL and VITE_PAYMENT_TOKEN_MINT.
• The repo has no mainnet SettlementSale program address, so the README uses a placeholder for it.

Test Plan

• Checked the documented defaults against src/config.ts and examples/shared/idl/settlement_sale.json
• Docs-only change; no code paths affected

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