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>

internal/web/static/favicon.svg

@@ -0,0 +1,8 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 64 64" width="64" height="64" role="img" aria-label="SelfPost">
+<!-- SelfPost · https://codeberg.org/mix/selfpost · AGPL-3.0
+     Wordmark set in IBM Plex Sans (OFL) and converted to outlines: the
+     file needs no font installed to render as drawn. -->
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internal/web/static/logo-compact.svg

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+<!-- SelfPost · https://codeberg.org/mix/selfpost · AGPL-3.0
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+     file needs no font installed to render as drawn. -->
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internal/web/static/panel.css

@@ -17,6 +17,13 @@ body {
 }
 main { max-width: 42rem; margin: 0 auto; }
 h1 { font-size: 1.4rem; margin: 0 0 1rem; }
+/* The full mark, on the two pages that have no navigation bar to carry the
+   compact one. It takes the column's width so its edges line up with the card
+   below it, capped at that column's own 24rem; height stays automatic because
+   the stamp's proportions are part of the mark and it is never scaled unevenly.
+   The subtitle stops resolving below 280px, which a 320px viewport still clears
+   once the body's padding is taken off. */
+.mark { display: block; width: 100%; max-width: 24rem; height: auto; margin-bottom: 1.4rem; }
 .card {
   background: #fff; border: 1px solid #e2e5e9; border-radius: 10px;
   padding: 1.5rem; margin: 0 auto;
@@ -58,6 +65,13 @@ main { max-width: 48rem; }
    every second line. They get a wider measure; every other page keeps the narrow
    one. The class comes from the layout, which stamps the page name onto <main>. */
 main.page-deliveries, main.page-mail_queue, main.page-system_log { max-width: 64rem; }
+/* The signed-out pages are a single card and nothing else, and .card.narrow
+   centres itself inside whatever holds it — so at the panel's usual width the
+   card floated in the middle while the mark and the heading stayed at the far
+   left, three alignments on a page with four elements. Narrowing the column to
+   the card's own width makes the three line up and puts the block as a whole in
+   the middle of the page. */
+main.page-login, main.page-setup { max-width: 24rem; }
 .card + .card { margin-top: 1.2rem; }
 .flash { background: #ecfdf3; border: 1px solid #abefc6; color: #067647; padding: 0.7rem 1rem; border-radius: 8px; margin-bottom: 1.2rem; }
 @media (prefers-color-scheme: dark) { .flash { background: #0d2818 !important; border-color: #1a5336 !important; color: #75d99b !important; } }
@@ -149,9 +163,21 @@ details form { margin-top: 0.6rem; }
 }
 @media (prefers-color-scheme: dark) { .nav { border-color: #2b3138 !important; } }
 .nav .links, .nav .session { display: flex; flex-wrap: wrap; gap: 0.2rem 0.9rem; align-items: center; }
-/* Pushed to the far edge so the two rows read as two blocks rather than as one
+/* The top row: mark at one edge, session block at the other. Splitting them to
+   the edges is what makes the two rows read as two blocks rather than as one
    ragged list that happened to wrap. */
+.nav .top { display: flex; align-items: center; justify-content: space-between; gap: 0.9rem; }
 .nav .session { justify-content: flex-end; }
+/* The mark is a link, but not one of the bar's entries: it takes none of the
+   padding and rounding the entry rule below applies, so its own edge lines up
+   with the page rather than sitting half a step inside it. The stamp carries
+   about 4px of field inside the file at this size, which is what puts it level
+   with the icons of the row underneath. */
+.nav .brand { padding: 0; }
+/* width/height are on the element too, so the row reserves the space before the
+   SVG has loaded; these keep the ratio if the box is ever squeezed. The stamp's
+   proportions are part of the mark — it is never scaled unevenly. */
+.nav .brand img { display: block; width: 110px; height: auto; }
 /* Each entry pairs an icon with its label, so the entry itself is a flex row
    rather than a run of text — that is also why the bar centres its items
    instead of aligning them on the text baseline. Account is included: it is a