fix: Send's balance line is drawn again when a refresh returns (closes #748)

Send opened at an address no refresh had read kept reading
"Current balance: unknown" after a refresh had read the balance, until the
screen was opened again. A token just added read the same way.

A refresh that returns while Send is up now runs updateSendBalance(). The
token list is not rebuilt, so the recipient, amount and token stay as the
user left them. README says so.

Model: opus-5-5

README.md

@@ -471,7 +471,9 @@ When "Back" returns to a screen that this page load has not drawn yet,
 Home is shown instead. "Back" onto a screen already drawn only unhides it, so
 unsaved input in Settings survives. Home is drawn again on every "Back". When a
 balance refresh returns while AddressDetail or AddressToken is showing, that
-screen draws its balances again.
+screen draws its balances again. When a refresh returns while Send is showing,
+Send draws its From address and current balance again. Its inputs and dropdown
+stay as the user left them.
 
 A network switch, or a change to "Use Blockscout", cuts the stack at the first
 screen that cannot be drawn from saved state. The cut also drops AddressToken