feat: a "Max" button on the Send screen (closes #198)

Max fills in a token's whole balance, or for ETH the exact balance minus
the fee reserve the confirmation screen's balance check gates on. The
confirmation screen works a max ETH amount out again from its own fee
estimate and signs it with that estimate's fee fields, so a fee that rose
before signing cannot push amount plus fee above the balance.
validateTransfer() still gates every send, the check that ETH covers a
token send's fee included. Where there is nothing to fill in, a flash
message says why.

Model: opus-5-5

docs/README.md

@@ -240,7 +240,8 @@ screen. Tokens can also be added from Settings, under "Tracked Tokens".
 2. Select what to send (ETH, or any ERC-20 token with a balance on this address
    that survives the spam filters).
 3. Enter the recipient address or ENS name (e.g. `vitalik.eth`).
-4. Enter the amount.
+4. Enter the amount, or click "Max" to fill in everything you can send: a
+   token's whole balance, or your ETH balance minus the network fee.
 5. Click "Review" to see the confirmation screen.
 
 The confirmation screen shows: