Kiosk: System Name entry in System Settings

Adds a System Name text-entry page (hint 'e.g. Living Room', 1-40 char
validation) and a data-driven System Settings row for it. Saving persists
locally and nudges the background sync; conflict/invalid results surface
the message and restore the previous name. Extends the settings-menu
tests.

src/settings_menu.h

@@ -9,6 +9,7 @@ namespace radiantwave::settings {
 enum class SystemSettingId {
     EmailAddress,
     LicenseKey,
+    SystemName,
     WiFiSetup,
     Timezone,
     DateFormat,