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

src/popup/views/helpers.js

@@ -128,7 +128,8 @@ function showView(name) {
 }
 
 // Create or update the debug/insecure warning banner.
-// Called on every view switch and after the settings debug toggle changes.
+// Called on every view switch, after the settings debug toggle changes, and
+// after a network switch in Settings.
 // The banner is shown when the compile-time DEBUG constant is true OR when
 // the user has enabled runtime debug mode via the settings easter egg, OR
 // when the active network is a testnet.