fix: store an absent explorer decimals as unknown instead of fabricating 18 (closes #349)

parseInt(decimals || "18") ran before writing stored tokenBalances[].decimals, so an explorer reporting no decimals produced a fabricated 18 indistinguishable from a real one at read time. That defeated the resolve-or-refuse guarantees of #306 and #340: their refusal paths were intact but never fired, because the guess was laundered upstream of them.

An absent scale is now stored as unknown, and a holding whose scale nothing knows carries a null balance -- unknown, never zero -- with six reader sites saying so rather than printing 0.0000. The Send screen resolves the display scale rather than reading the stored one, so a bundled token whose explorer row omits decimals still sends; when the scale cannot be resolved the stored quantity is withdrawn too, so the user is told the balance is unknown rather than only that the fee failed.

Existing fabricated 18s cannot be told apart retroactively and are replaced wholesale on the next balance refresh. An explorer-sourced scale stays trusted -- only fabrication is removed; the reasoning is recorded on the issue.

src/popup/views/addressToken.js

@@ -12,6 +12,7 @@ const {
     displaySymbol,
     truncateMiddle,
     balanceLine,
+    unknownableAmount,
     renderAddressHtml,
     attachCopyHandlers,
     goBack,
@@ -118,7 +119,9 @@ function show() {
             addr.tokenBalances,
             state.trackedTokens,
         );
-        amount = tb ? parseFloat(tb.balance || "0") : 0;
+        // null when the scale is unknown: no quantity to show, and none to
+        // price. balanceLine() states that rather than printing 0.0000.
+        amount = tb ? unknownableAmount(tb.balance) : 0;
         price = getPrice(symbol);
     }
 
@@ -152,7 +155,7 @@ function show() {
     attachCopyHandlers($("address-token-line"));
 
     // USD total for this token only
-    const usdVal = price ? amount * price : null;
+    const usdVal = price && amount !== null ? amount * price : null;
     const usdStr = formatUsd(usdVal);
     $("address-token-usd-total").innerHTML = usdStr || " ";