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

lib/core/src/app_keys.h

@@ -414,8 +414,9 @@ inline constexpr KeyHelp kSystemSystem[] = {
 // settings: Settings
 inline constexpr KeyHelp kSettings[] = {
     {"; .", "up, down"},
-    {"Enter", "edit, or open the page"},
+    {"Enter", "open the group or the page, or edit"},
     {", /", "change a switch or a slider"},
+    {"`", "back to the groups"},
 };
 
 // settings-choice: Settings, a choice