Mainnet indexer fix + retire testnet copy

- chain.js: derive the chain tip as the MAX height across the RPC pool and scan
  getLogs against that same synced node. Load-balanced public RPCs (publicnode)
  intermittently answer eth_blockNumber from a replica lagging thousands of
  blocks behind, which stalled the mainnet scan (latest < lastBlock, 0 events)
  and could skip freshly-mined events. This unblocks the live ledger + the
  purchase-confirmation email (both driven by the event scan).
- disclaimer: replace the "rehearsal/testnet" section with a live-on-Polygon
  real-money notice (it's live now).
- Point the dead Amoy fallbacks (purchase email tx link, contract-page source
  link) at Polygon mainnet / the verified mainnet contract.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

public/contract.html

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+<script src="/assets/contract.js?v=20260908p"></script>
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