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/chrome.html

@@ -48,4 +48,31 @@
 </div>
 {{end}}
 
+{{/* The brand mark, inline so it takes the page's ink and ember rather than the
+     fixed palette of /static/logo.svg (which the favicon needs). */}}
+{{define "mark"}}
+<svg class="mark" viewBox="0 0 200 172" aria-hidden="true">
+  <g fill="var(--ink)" stroke="currentColor" 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="currentColor" 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="var(--ember)">
+      <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>
+{{end}}
+
 {{define "newcount"}}<span class="count" id="new-count"{{if .OOB}} hx-swap-oob="true"{{end}}{{if not .NewCount}} hidden{{end}}>{{.NewCount}}</span>{{end}}