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

TODO.md

@@ -45,6 +45,17 @@ but the review is broader than any of them.
 
 # Completed Steps
 
+- 2026-10-05: A `holders_count` that is not a whole number in plain digits is
+  unknown, not read in part
+  ([#251](https://git.eeqj.de/sneak/AutistMask/issues/251)). `parseInt` read
+  `1,000` as 1, `0x10` as 0 and `1e3` as 1, a reported low count that hides the
+  token in the transaction history and the send-screen token selector. A count
+  above `Number.MAX_SAFE_INTEGER` is unknown too, not rounded or `Infinity`. The
+  balance list's `holders !== null` check, which did nothing, is dropped.
+  `README.md` and `docs/README.md` now say how each filter treats an unknown
+  count and that the token screen leaves out its "Holders:" row then, and
+  `README.md` lists `src/shared/holders.js`.
+
 - 2026-10-04: A popup boot in the tests loads transactions without failing
   ([#429](https://git.eeqj.de/sneak/AutistMask/issues/429)). The stand-in for
   `filterTransactions` in `tests/support/popupBoot.js` returned a bare list,