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/assets/contract.js

@@ -12,8 +12,8 @@
     $('lnkSource').href = src;
     $('lnkSource2').href = src;
   } else {
-    // private rehearsal chain: point at the audited Amoy verification instead
-    const src = 'https://repo.sourcify.dev/contracts/full_match/80002/0xaF47EC401A9B0D570a553b1A1c95D6079083636e/';
+    // fallback if explorer unset: point straight at the Sourcify-verified source
+    const src = 'https://repo.sourcify.dev/contracts/full_match/137/0xBE1ECA72AFF47d13D8E907e523F55eB9e2d365E0/';
     $('lnkExplorer').href = src;
     $('lnkSource').href = src;
     $('lnkSource2').href = src;