brand: put the mark on the README and in the panel

The stamp existed as files and appeared nowhere. It now opens the README, sits
at the left of the panel's navigation bar in its compact form, and stands above
the card on the two pages that carry no navigation — sign-in and first-run setup,
where it is the only thing telling an administrator which service just asked
them for a password. The tab icon moves to the stamp's small-size variant, so
the tab, the panel and the README stop being two different identities; the
earlier envelope drawing stays in docs/assets/icon.png, unreferenced.

The panel serves its own copies under internal/web/static/ because go:embed
cannot reach outside the package; the proof sheet says which file feeds which
slot. favicon.png is regenerated from the same icon at 32x32 for browsers with
no SVG favicon support, and is declared before the SVG so those that do take
the vector.

Sign-in and setup also get a column the width of their own card. Both are a
single .card.narrow, which centres itself while the heading above it stays at
the panel's left edge — with the mark added that would have been three
alignments on a page with four elements.

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

CHANGELOG.md

@@ -5,8 +5,27 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/); version
 
 ## [Unreleased]
 
+### Added
+
+- The project's mark is now in use rather than only on file. The README opens
+  with the full stamp; the panel carries the compact one at the left of its
+  navigation bar, linking to the status page, and the full one above the card
+  on the two pages that have no navigation — sign-in and first-run setup. The
+  browser tab icon changes with it, from the earlier envelope drawing to the
+  stamp's small-size variant, so the tab, the panel and the README are one
+  identity. The four brand files in `docs/assets/` had their wordmark converted
+  from live text to outlines: they were set in IBM Plex Sans, which is not
+  installed on the machines that render them, and the light/semibold contrast
+  between *Self* and *Post* — the whole of the mark — collapsed into whatever
+  fallback the viewer happened to have.
+
 ### Changed
 
+- panel: the sign-in and setup pages are now a column the width of their own
+  card. Both are a single narrow card, which centred itself while the heading
+  above it stayed at the panel's left edge; adding the mark would have made
+  that three alignments on a page with four elements.
+
 - panel: the three monitoring pages — Deliveries, Mail queue, System log — are
   now laid out wider (64rem against the 48rem the rest of the panel keeps).
   They carry data rather than prose: the send-log's seven columns had no room