ui: add jackal-scribe logo and favicons

Bundles the artifacto mascot as /static/logo.png (256×256),
/static/favicon-32.png and /static/apple-touch-icon.png. Header
shows the logo as a small rounded avatar; browser tabs get the
favicon. Replaces the .keep placeholder.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/server/templates/layout.html

@@ -4,6 +4,8 @@
 <meta charset="utf-8">
 <meta name="viewport" content="width=device-width, initial-scale=1">
 <title>{{ . }} · Artifacto</title>
+<link rel="icon" type="image/png" sizes="32x32" href="/static/favicon-32.png">
+<link rel="apple-touch-icon" sizes="180x180" href="/static/apple-touch-icon.png">
 <script src="https://cdn.tailwindcss.com"></script>
 <script src="https://unpkg.com/htmx.org@1.9.12"></script>
 <style>
@@ -19,8 +21,8 @@
 {{ define "header" }}
 <header class="border-b border-slate-800">
   <div class="max-w-6xl mx-auto px-6 py-4 flex items-center justify-between">
-    <a href="/" class="flex items-center gap-2 font-semibold text-lg">
-      <span class="inline-block w-2 h-2 rounded-full bg-indigo-400"></span>
+    <a href="/" class="flex items-center gap-2.5 font-semibold text-lg">
+      <img src="/static/logo.png" alt="" class="w-8 h-8 rounded-full ring-1 ring-slate-700">
       Artifacto
     </a>
     {{ if . }}