feat(ui): the key logo replaces the crown; favicon

The operator's concept, redrawn as a single-colour SVG in the accent gold:
a key whose bow is a cloverleaf holding three connected nodes, hanging on a
loop threaded through a keyring and swinging into the ring's middle, with a
small sparkle.

- static/logo/steward-logo.svg: the full mark (node graph in the bow).
- static/logo/steward-mark.svg: the small mark for 16-32 px (heavier lines,
  one centre node, two teeth); the node graph turns to mush below ~40 px.
  Used in the sidebar brand (was an inline crown) and as the favicon on every
  page and on the public share view.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

steward/static/steward.css

@@ -72,7 +72,7 @@ code { font-family: var(--fs-font-mono); font-size: 0.875em; background: var(--b
   display: flex; align-items: center; gap: 0.5rem;
   padding: 0.85rem 1.25rem; border-bottom: 1px solid var(--border);
 }
-.sidebar .brand svg { flex-shrink: 0; }
+.sidebar .brand img { flex-shrink: 0; }
 /* Only the nav list scrolls when long — the brand and user/logout stay pinned. */
 .side-nav { flex: 1; min-height: 0; overflow-y: auto; padding: 0.75rem 0; }
 .nav-group { margin-bottom: 0.9rem; }