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/CompassScreen.h

@@ -33,7 +33,6 @@ public:
     display.setColor(DisplayDriver::LIGHT);
     display.drawCenteredHeader("COMPASS");
 
-    const int hdr = display.headerH();
     const int cx  = display.width() / 2;
     const int ch  = display.getLineHeight();
     const int cw  = display.getCharWidth();
@@ -41,7 +40,7 @@ public:
     display.setTextSize(2);
     const int bigH = display.getLineHeight();
     display.setTextSize(1);
-    const int top      = hdr + 2;
+    const int top      = display.listStart();
     const int readout_y = display.height() - bigH - 1;   // size-2 readout baseline
     const int mid      = (top + readout_y) / 2;