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/gnss_service.h

@@ -16,7 +16,7 @@ namespace roro {
 
 // The GNSS receiver on the Cap LoRa-1262 (see CONTEXT.md and docs/milestones/M2.md): reads its
 // NMEA from the main loop's tick (about 450 bytes/s, so a 512-byte UART buffer covers a second),
-// holds the current Fix, and sets the clock from it. Settings → GNSS switches it on and off.
+// holds the current Fix, and sets the clock from it. Settings → GNSS and radio → GNSS switches it on and off.
 class GnssService : public Service {
    public:
     static constexpr int kRxPin = 15, kTxPin = 13;  // measured: M2 step 1