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a bundled token whose explorer row omits decimals still sends; when the scale cannot be resolved the stored quantity is withdrawn too, so the user is told the balance is unknown rather than only that the fee failed.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/shared/stateSchema.js

@@ -36,7 +36,13 @@
 //     indexed, assigned into, or .toLowerCase()'d — where a truthy value of
 //     the wrong type throws on the first read. The entries matter as much as
 //     the container: [1, 2] IS a list, and `t.address` is one level below the
-//     Array.isArray().
+//     Array.isArray(). What is checked on an ENTRY is the field the check
+//     exists for and no more — for trackedTokens and tokenBalances that is
+//     `address` alone; the rest of an entry is taken verbatim. So an entry's
+//     `decimals` and `balance` may be null, which is how balances.js records
+//     that nothing knows the token's scale
+//     (https://git.eeqj.de/sneak/AutistMask/issues/349), and every reader
+//     handles that null rather than being defended from it here.
 //   Container shape only: allowedSites, deniedSites. A falsy value or a list
 //     becomes {}; anything else is taken as stored and the entries are not
 //     checked.