fix(mobile): hide inline header nav below md; add overflow-x:clip guard

Header NavLinks (Top Stories/Canada/National/All) always rendered and
overflowed the ~360-400px viewport, causing whole-page horizontal
scroll + wrapped/bleeding active pill. Now hidden on phones (the
horizontally-scrolling SectionPills row under the header remains the
mobile nav); inline nav returns at md (768px). Added overflow-x:clip on
html as a global NOOP-safe against any future sideways overflow.

src/app/globals.css

@@ -8,6 +8,8 @@
 
 html {
   scroll-behavior: smooth;
+  /* never allow sideways scroll from any overflow (sticky-safe, unlike hidden) */
+  overflow-x: clip;
 }
 
 body {