fix(nav): drop nav view when target contact disappears; cap label entry

Review fixes:
- NearbyScreen: when the periodic detail refresh can't find the selected
  contact any more, clear _nav as well as _detail so the nav view and the
  input handler don't disagree about state.
- TrailScreen: bound the label keyboard to WAYPOINT_LABEL_LEN-1 (11) chars
  for both Mark-here and Rename, so the user can't type more than the stored
  label holds.

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

examples/companion_radio/ui-new/NearbyScreen.h

@@ -569,7 +569,7 @@ public:
           if (_entries[i].contact_idx == saved_contact_idx) { _sel = i; found = true; break; }
         }
       }
-      if (!found) _detail = false;
+      if (!found) { _detail = false; _nav = false; }  // contact gone — drop both views
       _detail_refresh_ms = millis();
     }