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/logo.svg

@@ -0,0 +1,23 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 200 172" role="img" aria-label="mangaBookmark">
+  <title>mangaBookmark</title>
+  <g fill="#100f0e" stroke="#f2ece5" stroke-width="5" stroke-linejoin="round" stroke-linecap="round">
+    <path fill="none" d="M28 36H4v114h192V36h-24"></path>
+    <path fill="none" d="M28 23H17v127h166V23h-11"></path>
+    <g id="mb-half">
+      <path d="M28 7 88 55v97L28 138z"></path>
+      <g fill="#f2ece5" stroke="none">
+        <path d="M37 25 55 39v41L37 66z"></path>
+        <path d="M60 42 79 57v42L60 84z"></path>
+        <path d="M37 75 79 108v13L37 88z"></path>
+        <path d="M37 98 79 129v11L37 131z"></path>
+      </g>
+    </g>
+    <use href="#mb-half" transform="matrix(-1 0 0 1 200 0)"></use>
+    <g stroke="#e0452c">
+      <path d="M100 4l9 5v11l-9 5-9-5V9z"></path>
+      <path d="M94 24h12v24H94z"></path>
+      <path d="M70 47h60v14H70z"></path>
+      <path d="M91 61h18v87l-9 20-9-20z"></path>
+    </g>
+  </g>
+</svg>

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;