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

The wallet's own Send screen renders the amount, the balance and the symbol
from the block explorer's cached decimals, but confirmTx encoded the transfer
from decimals() read off the contract at signing time and nothing compared the
two. A token whose on-chain scale disagrees with the cached one -- an
upgradeable or proxy token, a caller-dependent one, a stale or wrong explorer
entry, a compromised Blockscout -- therefore signed an amount that was never
displayed, off by a power of ten for every decimal place of disagreement. The
reproduction on the issue approves 0.25 and signs 250,000,000,000.

The scale is now carried forward on the pending transaction, taken from the
same tokenBalances entry the screen's own numbers come from, and the contract's
decimals() is read at signing time only to be compared with it. A disagreement
is a refusal that names both numbers, never a preference for either: both
candidate transfers move an amount nobody approved. New
src/shared/transferAmount.js holds that check, as the confirmTx counterpart to
approvalVerify.js, and takes the same stance on an absent or unusable value --
a quantity that cannot be compared with what was displayed has not been
checked. The gas estimate encodes from the same carried value and no longer
reads decimals() at all, so the estimate is for the transfer that would be
signed.

Nothing in the e2e suite had ever clicked #btn-confirm-send, so the popup's own
Send -> ConfirmTx -> Sign & Send -> WaitTx path had no coverage at all, which is
how this shipped. It is now driven end to end to a broadcast, with the
transfer() amount hand-decoded out of the raw signed bytes and asserted against
the amount read off the confirmation screen, plus a case where the fixture's
decimals() starts answering 18 after the screen was built and nothing reaches
eth_sendRawTransaction. The fixture gains that override and a receipt, so the
wait screen resolves to the success view instead of polling for the rest of the
run.

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**