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/balances.js

@@ -147,20 +147,20 @@ async function fetchTokenBalances(address, blockscoutUrl, trackedTokens) {
             // null is a holding of an amount that cannot be stated, which is
             // not the same as a holding of zero, and must never render as one.
             const bal = scale === null ? null : formatTokenBalance(raw, scale);
-            // null means the explorer reported no count, which is not the
-            // same as a count of zero.  This gate is not the low-holder
-            // display filter: it has no user-facing off switch and governs
-            // the whole balance list, so it stays strict and admits a token
-            // only on a reported count — an unreported one is no evidence.
-            // A legitimate token still reaches the list through the known
+            // null means the explorer reported no readable count, which is
+            // not the same as a count of zero.  This gate is not the
+            // low-holder display filter: it has no user-facing off switch and
+            // governs the whole balance list, so it stays strict and admits a
+            // token only on a reported count — an unreported one is no
+            // evidence, and `null >= LOW_HOLDER_THRESHOLD` is false.  A
+            // legitimate token still reaches the list through the known
             // token list or by the user tracking it, and the null is carried
             // through to the views, where the two low-holder filters treat
             // an unknown count as "do not judge" rather than as zero.
             const holders = parseHoldersCount(item.token.holders_count);
             const isKnown = TOKEN_BY_ADDRESS.has(tokenAddr);
             const isTracked = trackedSet.has(tokenAddr);
-            const hasEnoughHolders =
-                holders !== null && holders >= LOW_HOLDER_THRESHOLD;
+            const hasEnoughHolders = holders >= LOW_HOLDER_THRESHOLD;
 
             // Skip spam tokens the user never asked to see
             if (!isKnown && !isTracked && !hasEnoughHolders) continue;