feat(ui): brand mark logo in the web UI, panel, and favicon

The mark is the same drawing in three places with three different
colour sources: /static/logo.svg carries a fixed palette because a
favicon has no page to inherit from, the {{define "mark"}} block takes
--ink/--ember so it flips with the theme, and the userscript copy lives
inside the shadow root next to the tokens it uses.

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

backend/static/style.css

@@ -197,14 +197,21 @@ button { cursor: pointer; }
 /* ---- brand + chrome ---- */
 .brand {
   margin: 0;
+  display: flex;
+  align-items: center;
+  gap: 10px;
   font: 400 26px/1 var(--font-display);
   color: var(--paper);
 }
 .brand em { color: var(--ember); font-style: italic; }
+.brand .mark { width: 30px; height: 26px; flex: none; overflow: visible; }
+/* The line art is drawn at a 5px stroke on a 200-unit grid; at brand size that
+   thins out, so it is nudged up rather than scaled down blindly. */
+.brand .mark g { stroke-width: 6.5; }
 
 .topbar {
   display: flex;
-  align-items: baseline;
+  align-items: center;
   justify-content: space-between;
   gap: 16px;
   padding: 22px 20px 14px;
@@ -745,6 +752,7 @@ button { cursor: pointer; }
   :root { --cover-w: 80px; --row-gap: 20px; }
   .topbar { padding: 26px 32px 18px; }
   .brand { font-size: 30px; }
+  .brand .mark { width: 35px; height: 30px; }
 
   .chrome {
     flex-direction: row;