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

README.md

@@ -695,6 +695,7 @@ src/
     balances.js       — ETH + ERC-20 balance fetching via RPC + Blockscout
     constants.js      — chain IDs, default RPC endpoint, ERC-20 ABI
     ens.js            — ENS forward/reverse resolution (popup only)
+    holders.js        — holder-count parsing and the low-holder rule
     prices.js         — ETH/USD and token/USD via CoinDesk API
     scamlist.js       — known fraud contract addresses
     state.js          — persisted state (extension storage)
@@ -1123,13 +1124,18 @@ claiming a symbol that belongs to the native asset and therefore has no
 legitimate contract at all (`"ETH"`, and every network's `nativeCurrency`, such
 as `"SepoliaETH"`, on every network). That filter is unconditional — the "Hide
 tokens with fewer than 1,000 holders" setting governs the transaction history
-and the send-screen token selector, not this list. `fetchTokenBalances()` stores
-every nonzero holding of a token it admits, however small, but a holding below
-0.000001 is left out of the balance lists, the send-screen token selector, the
-address total and the remove-address warning (`isBelowOneMillionth()` in
-`src/shared/amountDisplay.js`). The Send and confirmation screens show it when
-its token is the one being sent. Tracked tokens with a zero balance are listed
-as well while "Show tracked tokens with zero balance" is on.
+and the send-screen token selector, not this list. A token's holder count is
+unknown when the explorer reports none, or reports anything other than a whole
+number written in digits alone, such as `1,000` or `1e3` (`parseHoldersCount()`
+in `src/shared/holders.js`). This list does not take an unknown count as 1,000
+or more, so such a token is shown only when it is on the bundled list or
+tracked. `fetchTokenBalances()` stores every nonzero holding of a token it
+admits, however small, but a holding below 0.000001 is left out of the balance
+lists, the send-screen token selector, the address total and the remove-address
+warning (`isBelowOneMillionth()` in `src/shared/amountDisplay.js`). The Send and
+confirmation screens show it when its token is the one being sent. Tracked
+tokens with a zero balance are listed as well while "Show tracked tokens with
+zero balance" is on.
 
 #### Stored state and its version
 
@@ -1438,7 +1444,9 @@ view would leave a wallet one click from deletion.
     - Send / Receive buttons
     - Token contract well (ERC-20 only): full contract address (tap to copy,
       etherscan link) plus name, symbol, decimals, holder count and project
-      website where known
+      website where known. The "Holders:" row is left out, not shown as 0, when
+      the token's balance-list entry has no holder count: the explorer did not
+      report a readable one, or the token is not in the balance list
     - Token-filtered transaction list (only this token's transfers)
 - **Transitions**:
     - "Send" → **Send** (token locked: the dropdown is replaced by a static
@@ -2389,7 +2397,8 @@ indexes it as a real token transfer.
   fewer than 1,000 holders are hidden from transaction history by default.
   Legitimate tokens have substantial holder counts; poisoning tokens typically
   have zero. This catches new poisoning contracts that use novel symbols not in
-  the known token list.
+  the known token list. A transfer whose token's holder count is unknown (see
+  Data Model) is kept: only a reported count below 1,000 hides it.
 
 - **Fraud contract blocklist**: AutistMask maintains a local list of known fraud
   contract addresses. Token transfers involving these contracts are filtered
@@ -2399,7 +2408,9 @@ indexes it as a real token transfer.
 - **Send-side token filtering**: Tokens with fewer than 1,000 holders are
   excluded from the token selector on the send screen. This prevents users from
   accidentally interacting with a spoofed token that appeared in their balance
-  via a fake Transfer event.
+  via a fake Transfer event. A token whose holder count is unknown is kept in
+  the selector. The selector offers only tokens in the balance list, so such a
+  token is one on the bundled list or one the user tracks.
 
 - **Dust transaction filtering**: A second wave of the same attack used real
   native ETH transfers instead of fake tokens. Transaction
@@ -2423,8 +2434,9 @@ indexes it as a real token transfer.
   both cases identically to the history. The fraud contract blocklist is applied
   unconditionally on that selector and is not consulted by the balance list at
   all. The low-holder setting also gates the send selector, while the balance
-  list's own 1,000-holder floor is unconditional (see Data Model). The dust
-  threshold applies to the transaction history alone.
+  list's own 1,000-holder floor is unconditional (see Data Model). An unknown
+  holder count passes the history and send-selector filters but not that floor.
+  The dust threshold applies to the transaction history alone.
 
 #### Phishing Domain Protection