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

TODO.md

@@ -45,6 +45,18 @@ 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, or for ETH the exact
+  balance minus the fee reserve that check gates on, never the four-decimal
+  balance shown. 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