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>

src/helpers/ui/DisplayDriver.h

@@ -39,7 +39,10 @@ public:
   // Use these instead of hardcoded pixel values so layouts adapt to any display.
   int lineStep()             const { return getLineHeight() + 2; }         // row pitch: text + gap
   int headerH()              const { return getLineHeight() + 3; }         // title bar height
-  int listStart()            const { return headerH(); }                   // y where list items begin
+  // y where list items begin: a 2px breathing gap below the header separator so
+  // the first row doesn't touch the line (matches the hand-rolled hdr+2 used by
+  // the graphical screens).
+  int listStart()            const { return headerH() + 2; }
   int listVisible(int itemH) const { return (height() - listStart()) / itemH; }
   int listVisible()          const { return listVisible(lineStep()); }
   // x where a right-side value column starts (leaves ~8 chars for the value)