refactor: dedup persistence header, harden action menu, tidy small items

- Persist.h: shared writeHeader()/readHeader() for the magic+version+count
  on-disk header; Waypoint and Trail snapshots now share one implementation
  (byte layout unchanged; each keeps its own count clamp/reject + extra fields).
- TrailScreen: pushAction() guards _act_map / PopupMenu capacity, so adding a
  new action can't silently overrun the fixed array.
- PopupMenu: add count()/setSelected() accessors; replace the remaining direct
  _sel/_count field pokes in NearbyScreen, TrailScreen and QuickMsgScreen.
- Trail summary: unit-suffixed time ("1h 05m" / "5m 03s") so hours can't be
  misread as minutes.
- Trail::haversineMeters delegates to geo::haversineKm — single Haversine
  implementation (also trims a little flash).

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/PopupMenu.h

@@ -94,4 +94,6 @@ struct PopupMenu {
   }
 
   int selectedIndex() const { return _sel; }
+  int count() const { return _count; }
+  void setSelected(int i) { if (i >= 0 && i < _count) _sel = i; }
 };