fix: Send refuses every amount the confirmation screen would refuse (closes #532)

Send accepted any amount parseFloat() read as above zero, so "1e-3",
"1abc" and an amount with more decimal places than the token has reached
ConfirmTx, which then refused them. isValidAmount() in
src/shared/txValidation.js is now the one check: Review and Max on Send
call it with the token's decimal places, and validateTransfer() calls it
for ConfirmTx, which now also refuses a zero amount and a token amount
with more places than the token has.

Model: opus-5-5

README.md

@@ -1686,7 +1686,10 @@ view would leave a wallet one click from deletion.
 - **Transitions**:
     - "Review" (valid inputs, ENS resolved) → **ConfirmTx**
     - "Review" with an unresolvable ENS name or an invalid amount → flash
-      message, no screen change
+      message, no screen change. An amount is valid when it is a number above
+      zero written as digits with at most one decimal point, with no more
+      decimal places than the token has and never more than 18 (ETH's own),
+      which is also the check ConfirmTx makes before anything can be sent
     - "Back" → previous screen (Home, AddressDetail, or AddressToken)
 
 #### ConfirmTx (`confirm-tx`)