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/gnss_app.cpp

@@ -76,7 +76,7 @@ void GnssApp::draw(Canvas& c) {
         c.drawString(held ? "GNSS is paused" : "GNSS is off", 4, area.y + 4);
         c.setFont(&fonts::body);
         c.setTextColor(theme::kMuted);
-        c.drawString(held ? "The LoRa radio is listening, and" : "Settings > GNSS turns it on.", 4, area.y + 22);
+        c.drawString(held ? "The LoRa radio is listening, and" : "On: Settings > GNSS and radio > GNSS", 4, area.y + 22);
         if (held) c.drawString("Settings pauses GNSS for it.", 4, area.y + 22 + theme::kLineHeight);
         return;
     }