harden: a holders_count that is not plain digits is unknown, not read in part (closes #251)

parseHoldersCount used parseInt, which reads "1,000" as 1, "0x10" as 0 and
"1e3" as 1: a reported low count, which hides the token in the transaction
history and the send-screen token selector. It now accepts only a whole
number of zero or more, or a string of digits alone, no larger than
Number.MAX_SAFE_INTEGER, and returns null for anything else. The balance
list's holders !== null check did nothing, since null >= 1000 is already
false, and is dropped. README.md and docs/README.md say how each filter
treats an unknown count and that the token screen then leaves out its
Holders row; README.md lists src/shared/holders.js.

Model: opus-5-5

src/shared/transactions.js

@@ -137,9 +137,10 @@ function parseTokenTransfer(tt, addrLower, chainId) {
         contractAddress: normalizeAddress(
             tt.token?.address_hash || tt.token?.address || "",
         ),
-        // null when the explorer reported no count: unknown, not zero.  The
-        // low-holder filter declines to judge a null, so a legitimate token
-        // is not hidden because a field went missing upstream.
+        // null when the explorer reported no readable count: unknown, not
+        // zero.  The low-holder filter declines to judge a null, so a
+        // legitimate token is not hidden because a field went missing
+        // upstream.
         holders: parseHoldersCount(tt.token?.holders_count),
         chainId: chainId,
     };