Settings: its rows in five groups

Settings had grown to 21 rows in one list. It opens on five groups (This
device, Display, GNSS and radio, Network, System); Enter opens one, Back
returns to the groups, and Back from the groups leaves Settings.

SettingsMenu holds the groups and which one is open (host-tested: every
setting is in exactly one group). The guide, the how-tos, the README,
the scripts' messages and the firmware's own name the new paths, such as
"Settings > System > Firmware".

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0162FokPdvY2KsS4NBfwyWPk

src/services/update_service.cpp

@@ -284,7 +284,7 @@ void UpdateService::serve() {
             if (r == Request::BackgroundCheck && gitea_.latest(latest) &&
                 release::shouldAnnounce(latest, runningVersion(), failedVersion(), announced_)) {
                 announced_ = latest.tag;
-                notify(latest.tag + " is out: see Settings > Firmware", NotificationLevel::Info);  // 48 bytes at most
+                notify(latest.tag + " is out: Settings > System > Firmware", NotificationLevel::Info);  // 48 bytes at most
             }
             break;
         }