fix(ui): gap below header separator, one method everywhere

Content drawn under the title separator touched the line on every standard
screen, because listStart() == headerH() (the row right after the separator).
Graphical screens worked around it with a hand-rolled hdr+2.

Bake a 2px breathing gap into listStart() so every list gains it at once, and
switch the screens that hand-rolled the offset (Compass, Nav, Nearby detail) to
listStart() so the content top is computed the same way everywhere.

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

examples/companion_radio/ui-new/NavView.h

@@ -37,7 +37,7 @@ inline void draw(DisplayDriver& d,
   geo::fmtDist(dist, sizeof(dist), dist_km, imperial);
 
   const int step = d.lineStep();
-  int y = hdr + 2;
+  int y = d.listStart();
 
   // Distance — emphasised at size 2.
   d.setTextSize(2);