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

README.md

@@ -1467,6 +1467,17 @@ 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 a
+      token's balance, cut down to the 18 decimal places ConfirmTx accepts for a
+      token that has more, 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; an estimate that finishes after the screen was
+      left or the address, holding or recipient changed fills nothing in. 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 +1494,14 @@ 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",