Keep mobile article navigation at the bottom (#8833)

This keeps the mobile article navigation fixed to the bottom of the viewport by clearing any inherited top offset in the small-screen rule. It updates both LTR and RTL base theme CSS so the previous/next bar remains anchored while browsing on mobile Safari.

Verification: targeted CSS assertion for both base theme files; `git diff --check`.

Closes #8829

Co-authored-by: mzl2233 <mzl2233@users.noreply.github.com>

p/themes/base-theme/frss.css

@@ -2867,6 +2867,7 @@ html.slider-active {
 
 	#nav_entries {
 		display: flex !important;
+		top: auto;
 		width: 100%;
 	}
 

p/themes/base-theme/frss.rtl.css

@@ -2867,6 +2867,7 @@ html.slider-active {
 
 	#nav_entries {
 		display: flex !important;
+		top: auto;
 		width: 100%;
 	}