feat(panel): navigation as a left column, with a section index on the long pages

The navigation was a bar across the top that did not fit on one row — six page
entries and the session block against the panel's width — and had to be split
into two, costing the top of every page. It is now a column down the left edge:
one left edge to scan, the current entry marked down its leading edge, sticky so
it stays in view, and room under the entries for the current page's own
sections. Below the width the two columns need it lies back down into the same
wrapping rows as before; six entries need no drawer.

The section index is for the two pages long enough to need one — the domain page
(nine cards) and the status page (eight). Each card carries an id and the page's
template defines the list by overriding an empty "sections" block in the layout,
so a page that defines nothing renders no index. panel.js marks the section in
view, looking targets up by id on each pass so the status page swapping its
cards out every five seconds cannot leave it measuring boxes that have left the
document; the links themselves are plain fragment links and need no script.

Verified against the real pages rendered by a local panel at 1300px, 924px and
481px wide.

CHANGELOG.md

@@ -21,6 +21,30 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/); version
   — no `/proc` outside Linux — leave the card in place showing "unknown". The
   usage bars are `<meter>` elements: the panel's CSP has no inline-style
   exemption, so a bar's length has to travel on an attribute.
+- An index of the current page's own sections in the navigation column, for the
+  two pages long enough to need one: the domain page (nine cards, from the DNS
+  records to publish down to the danger zone) and the status page (eight). Each
+  card carries an id and the page's template defines the list
+  (`{{define "sections"}}`); every other page defines nothing and shows no
+  index. `panel.js` marks the section currently in view, looking its targets up
+  by id on each pass so the status page swapping its cards out every five
+  seconds cannot leave it measuring boxes that have left the document. The
+  links are plain fragment links and work with JavaScript blocked; only the
+  highlight needs it.
+
+### Changed
+
+- The panel's navigation is a column down the left edge instead of a bar across
+  the top. As a bar it did not fit on one row — six page entries and the
+  session block against the panel's width — and had to be split into two,
+  costing the top of every page; standing it up removes the compromise, gives
+  the entries one left edge to scan down, and leaves room under them for the
+  section index above. It is sticky, so both lists stay in view on the long
+  pages, and the current entry is marked down its leading edge rather than
+  underlined. Below the width the two columns need, it lies back down into the
+  wrapping rows it used to be — no drawer and no hamburger, since six entries
+  fit. The markup now lists the blocks in the order they are drawn, so the tab
+  order follows the eye instead of starting at Sign out.
 
 ## [0.5.0] - 2026-08-06