fix: a network switch in Settings updates the testnet banner at once (closes #511)

The Settings network handler now redraws the red banner once the switch
is saved, so it appears on a switch to Sepolia and goes on a switch back
to mainnet while Settings is still on screen. Before, the banner changed
only when the next screen came up. The Chrome browser suite switches
both ways on Settings and reads the banner each time.

Model: opus-5-5

README.md

@@ -418,6 +418,10 @@ Each group of these assertions records a coverage key and a final case demands
 the exact set, so a section that silently stopped running reddens the suite
 instead of quietly shrinking it.
 
+It also covers the red banner on the Settings screen: switching the network
+there to Sepolia must put up the `[TESTNET]` banner, and switching back to
+mainnet must take it down, both while Settings is still on screen.
+
 It also covers the confirmation screen, for both a native ETH send and an ERC-20
 send: Send disabled while the fee estimate is in flight, enabled once it lands,
 the fee block quoting the expected cost and the reserve separately, the distinct