refactor(admin): make Admin remote-only, move device settings to Settings

A device administering itself via the Admin tool read as awkward, so the
local mode is gone and its options relocated to Settings.

- AdminScreen is remote-only: drop the CHOOSER phase, _local_mode, the
  LocalField/LocalKind model and activateLocalField/commitLocalField.
  Phases collapse to LOGIN/COMMAND/REPLY, entered only via startFor().
- Tools > Admin opens straight to the Nodes picker (ACT_ADMIN ->
  pickAdminTarget()). Backing out of a command screen returns to that
  picker; the picker's Cancel returns to Tools. gotoAdminScreen() removed.
- Settings > System gains Name (keyboard-edited node_name) and Reboot
  (action row, placed last so it isn't the default cursor). Radio + TX
  power were already in Settings; Send advert is the home ADVERT page.

Docs: tools_screen.md Admin section rewritten remote-only; settings_screen.md
System table gains Name + Reboot.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

examples/companion_radio/ui-new/UITask.cpp

@@ -1469,7 +1469,6 @@ void UITask::begin(DisplayDriver* display, SensorManager* sensors, NodePrefs* no
 void UITask::gotoSettingsScreen()  { setCurrScreen(settings); }
 void UITask::gotoToolsScreen()     { setCurrScreen(tools_screen); }
 void UITask::gotoBotScreen()       { setCurrScreen(bot_screen); }
-void UITask::gotoAdminScreen()     { setCurrScreen(admin_screen); }  // AdminScreen::onShow() resets it
 void UITask::gotoNearbyScreen()    { setCurrScreen(nearby_screen); }
 
 void UITask::pickAdminTarget() {