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/templates/login.html

@@ -6,6 +6,7 @@
   <meta name="viewport" content="width=device-width, initial-scale=1, viewport-fit=cover">
   <meta name="color-scheme" content="dark light">
   <title>mangaBookmark</title>
+  <link rel="icon" href="/static/logo.svg" type="image/svg+xml">
   <link rel="stylesheet" href="/static/style.css">
   <link rel="preload" href="/static/fonts/instrument-serif-400-latin.woff2" as="font" type="font/woff2" crossorigin>
 </head>
@@ -13,7 +14,7 @@
   <main class="login-card">
     <div>
       <span class="eyebrow">Private library</span>
-      <h1>manga<em>Bookmark</em></h1>
+      <h1 class="brand">{{template "mark" .}}<span>manga<em>Bookmark</em></span></h1>
     </div>
     <form method="post" action="/login">
       <div>