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

Max fills in a token's whole 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 or
recipient 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

TODO.md

@@ -45,6 +45,20 @@ but the review is broader than any of them.
 
 # Completed Steps
 
+- 2026-10-05: The Send screen has a "Max" button
+  ([#198](https://git.eeqj.de/sneak/AutistMask/issues/198)). Emptying an ETH
+  address took guessing an amount and being refused by the confirmation screen's
+  balance check. Max fills in a token's whole balance, cut to the 18 decimal
+  places the confirmation screen accepts, or for ETH the exact balance minus the
+  fee reserve that check gates on, never the four-decimal balance shown; a fee
+  estimate that finishes after the Send screen was left, or its address, holding
+  or recipient 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: fetched again at signing, a fee that had risen since
+  would leave amount plus fee above the balance, and the node would refuse the
+  send. A token's maximum is still refused when ETH cannot pay the fee. Where
+  there is nothing to fill in, a flash message says why.
+
 - 2026-10-05: A token scale of zero decimals is tested
   ([#325](https://git.eeqj.de/sneak/AutistMask/issues/325)).
   `resolveTokenDecimals()` already used a scale of 0 from the bundled list or