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

README.md

@@ -1467,6 +1467,15 @@ view would leave a wallet one click from deletion.
     - Amount input with current balance display, which reads
       `Current balance: unknown (SYMBOL)` for a token whose scale is unknown, as
       ConfirmTx's balance line does (see Unknown token scale)
+    - "Max" button beside the amount input, always in place. It fills in the
+      most the selected holding can send: a token's whole balance, or for ETH
+      the exact balance minus the network fee reserve that ConfirmTx's balance
+      check gates on, never the rounded balance shown above it. The ETH fee is
+      estimated for the recipient entered, so it asks for a recipient first.
+      Where there is nothing to fill in, a flash message says why: the balance
+      does not cover the fee, the fee could not be estimated, or the token's
+      balance is unknown or zero. Typing in the amount makes it an ordinary
+      amount; changing what to send clears an amount Max filled in
     - "Review" button, disabled until the recipient validates
 - **Transitions**:
     - "Review" (valid inputs, ENS resolved) → **ConfirmTx**
@@ -1483,7 +1492,14 @@ view would leave a wallet one click from deletion.
     - Token contract: full address + etherscan link (ERC-20 only)
     - From: blockie + color dot + full address + etherscan link + wallet title
     - To: blockie + color dot + full address + etherscan link + ENS name
-    - Amount: value + symbol (USD in parentheses)
+    - Amount: value + symbol (USD in parentheses). An ETH amount Send's "Max"
+      filled in is worked out again from this screen's own fee estimate when it
+      arrives, as the balance minus the reserve, and the transaction is signed
+      with that estimate's fee fields, so a fee fetched again at signing cannot
+      exceed what the amount leaves behind. The address keeps whatever part of
+      the reserve the transaction does not use. If the balance no longer covers
+      the fee, the amount is left as it was and the amount-plus-fee error below
+      blocks the send
     - Your balance: value + symbol (USD in parentheses), or `unknown (SYMBOL)`
       for a token whose scale is unknown
     - Network fee: "Estimating..." then two lines, or "Unable to estimate",