refactor(ui): extract shared helpers, dedup screens, GPS shutdown via provider

Pull repeated UI idioms into DisplayDriver and remove duplicated logic:

- DisplayDriver: add drawScrollArrows() and drawInvertedHeader(); replaces 11
  copy-pasted scroll-arrow blocks across 5 screens and 3 inverted title-bar
  blocks (NearbyScreen x2, NavView). Standardises the detail-view separator.
- useImperial(): single source on UITask; NearbyScreen/TrailScreen delegate
  instead of each re-reading NodePrefs.units_imperial.
- NearbyScreen: extract saveSelectedWaypoint() (two identical blocks -> one);
  drop the redundant local label buffer (WaypointStore truncates).
- UITask::shutdown(): power GPS off through LocationProvider::stop() instead of
  poking PIN_GPS_EN directly — centralises enable/reset-pin + active-level logic.

Net -58 lines. Verified: GAT562 (SSD1306) and WioTrackerL1Eink (GxEPD) solo
builds compile clean.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

examples/companion_radio/ui-new/UITask.h

@@ -135,6 +135,8 @@ public:
   void onBLEDisconnected() override { _next_refresh = 0; }
 
   NodePrefs* getNodePrefs() const { return _node_prefs; }
+  // Global metric/imperial preference for distance/speed display.
+  bool useImperial() const { return _node_prefs && _node_prefs->units_imperial; }
   uint16_t getBattMilliVolts() const { return _batt_mv > 0 ? _batt_mv : AbstractUITask::getBattMilliVolts(); }
   void gotoHomeScreen() { setCurrScreen(home); }
   void gotoSettingsScreen();