style(panel): give the navigation column's mark the column's full width

The mark kept the 110px it had as part of the bar, which was all there was
room for in a row; in a column it ended halfway across, sharing an edge with
nothing below it. At the column's width its edges line up with the page
entries, and the SVG's own inset puts the drawn mark on the entries' icons.
The width/height attributes carry the file's real proportions so the column
still reserves the right height before the SVG loads.

Where the column lies back down into a bar, the mark returns to the compact
size: at full width it would take a row to itself above the entries.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

CHANGELOG.md

@@ -58,6 +58,13 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/); version
   flush against the field above, and Save limit and Remove limit — two posts,
   hence two forms — share one row, the first button bound to its form by the
   `form` attribute rather than by sitting inside it.
+- The mark at the head of the navigation column takes the column's full width
+  instead of the 110px it kept from the bar. In a row that size was all there
+  was room for; in a column it left the mark ending halfway across, with no
+  edge shared with anything below it. At the column's width its edges line up
+  with the page entries under it, as the full mark already does with the card
+  beneath it on the signed-out pages. Where the column lies back down into a
+  bar it returns to the compact size, which is what fits beside the entries.
 
 ## [0.5.0] - 2026-08-06