fix: sign the ERC-20 amount the confirmation screen displayed (closes #305)

The send screen was built from the indexer's decimals while the transfer was
encoded from the contract's decimals() read at signing time, with nothing
comparing them. A token whose scales disagree moved 10^12 times the approved
amount.

The displayed scale is now carried on pendingTx from the same tokenBalances
entry the amount, balance and symbol were rendered from, and both encode sites
use it. transferAmount.js refuses rather than falling back when the two scales
disagree or either is unusable.

Adds the first end-to-end coverage of the popup's own Send -> ConfirmTx ->
Sign & Send path; #btn-confirm-send had never been clicked by any test.

README.md

@@ -992,6 +992,13 @@ on ConfirmTx, DeleteWallet, ApproveTx and ApproveSign.
 - **Transitions**:
     - "Sign & Send" (correct password) → broadcast tx → **WaitTx**
     - "Sign & Send" (correct password) → broadcast fails → **ErrorTx**
+    - "Sign & Send" on an ERC-20 whose contract answers `decimals()` with a
+      different number than the amount above was displayed with → nothing is
+      signed → **ErrorTx** naming both numbers. The transfer is encoded from the
+      decimals the screen rendered, carried forward on the pending transaction;
+      the contract's own answer is read at signing time only to be compared with
+      it, and a disagreement is a refusal rather than a preference for either
+      value (`src/shared/transferAmount.js`)
     - "Sign & Send" (wrong password) → "Wrong password." on the password error
       line, no screen change
     - "Back" → **Send**