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/apps/firmware_page.h

@@ -18,7 +18,7 @@
 
 namespace roro {
 
-// Settings → Firmware: the running version and its Probation, where to push Firmware Updates, the
+// Settings → System → Firmware: the running version and its Probation, where to push Firmware Updates, the
 // project's releases on Gitea (the latest, and the ten before it), and the Update Files on the SD
 // card (in /updates) to install from.
 class FirmwarePage {