fix(panel): lay every page out in one column width

The page column was the 48rem reading measure, which the send log, the mail
queue and the system log widened to 64rem for their tables. The navigation and
the page are centred as a pair, so that difference did not only change the
page's width: it moved the navigation column and the left edge of every card
on the way between two pages. The column is now 64rem throughout and the
measure lives inside it — a page's heading, cards, back link and version footer
are held to 48rem and centred in the column, and the pages made of data opt out
and take the column whole.

Which pages those are is declared by the page (a "wide" block in its template,
the same mechanism as the section index) rather than derived from the
navigation entry: .Active cannot tell the send log from a single delivery's
page, which is prose and now keeps the measure instead of inheriting the log's
width.

Two rules follow from centring the children rather than the cards. The heading
takes its margins as longhands, since the shorthand re-zeroed the auto side
margins and pinned it a measure's width left of the card under it; and the back
link is a block, since auto margins centre a block and do nothing for an
inline-block.

The scrollbar's width is reserved on every page as well. Without it a short
page and a long one are laid out in viewports differing by that width, which
moved the same things again, in the same direction, for a second reason.

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

CHANGELOG.md

@@ -52,6 +52,22 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/); version
 
 ### Changed
 
+- Every panel page is laid out in one column of the same width, so moving
+  between them no longer shifts the navigation and the cards sideways. The
+  column used to be the 48rem reading measure, which the send log, the mail
+  queue and the system log widened to 64rem for their tables — and since the
+  navigation and the page are centred as a pair, that difference moved
+  everything on screen on the way between two pages. The column is now 64rem
+  throughout, with the reading measure kept inside it: a page's heading, cards,
+  back link and version footer are held to 48rem and centred in the column,
+  and the three pages made of data opt out and take the column whole. Which
+  pages those are is declared by the page itself (a `wide` block in its
+  template, the same mechanism as the section index) rather than derived from
+  the navigation entry, so a single delivery's page — prose, but filed under
+  the send log — keeps the measure. The scrollbar's width is now reserved on
+  every page as well: without it a short page and a long one were laid out in
+  viewports differing by that width, which moved the same things again.
+
 - The mark's small-size variant — the tab icon and the `SP` initials it
   carries — sets its S in Medium where the wordmark sets it in ExtraLight.
   Against the P's SemiBold the ExtraLight S is a 0.90 stem against 3.40,