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

Max fills in a token's balance, cut down to the 18 decimal places the
confirmation screen accepts, or for ETH the exact balance minus the fee
reserve the confirmation screen's balance check gates on. An ETH fee estimate
that finishes after the Send screen was left, or its address, holding,
recipient or amount changed, fills nothing in. 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 +240,9 @@ 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 it in: a token's balance, cut to 18
+   decimal places, or your ETH balance minus the amount reserved for the network
+   fee.
 5. Click "Review" to see the confirmation screen.
 
 The confirmation screen shows: