feat(ui): incorporate PubScript logo as favicon & Topbar mark

Recreate the gold-rimmed 'PS' typewriter-tile emblem as scalable SVGs:
- public/logo.svg — dark-ground emblem used as the app favicon
- public/logo-mark.svg — transparent mark placed in the Topbar beside
  the page title, with a subtle hover scale

Add <link rel=icon> to index.html and a .ps-topbar-logo rule in
AppShell.css.

apps/web/index.html

@@ -6,6 +6,7 @@
     <meta name="color-scheme" content="dark light" />
     <title>PubScript</title>
     <meta name="description" content="A local-first writing environment for authors." />
+    <link rel="icon" type="image/svg+xml" href="/logo.svg" />
   </head>
   <body>
     <div id="root"></div>

apps/web/public/logo-mark.svg

@@ -0,0 +1,37 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512" width="512" height="512" role="img" aria-label="PubScript">
+  <defs>
+    <linearGradient id="mg-gold" x1="0" y1="0" x2="0" y2="1">
+      <stop offset="0" stop-color="#f9edc4"/>
+      <stop offset="0.27" stop-color="#e8c76a"/>
+      <stop offset="0.52" stop-color="#c9a23a"/>
+      <stop offset="0.72" stop-color="#f4d683"/>
+      <stop offset="1" stop-color="#937420"/>
+    </linearGradient>
+    <radialGradient id="mg-ivory" cx="45%" cy="38%" r="72%">
+      <stop offset="0" stop-color="#fffdf4"/>
+      <stop offset="0.72" stop-color="#f5ecd6"/>
+      <stop offset="1" stop-color="#e8dcbd"/>
+    </radialGradient>
+    <filter id="mg-shadow" x="-35%" y="-35%" width="170%" height="170%">
+      <feDropShadow dx="0" dy="5" stdDeviation="8" flood-color="#000000" flood-opacity="0.35"/>
+    </filter>
+  </defs>
+
+  <g filter="url(#mg-shadow)">
+    <!-- P tile -->
+    <circle cx="126" cy="256" r="120" fill="url(#mg-gold)"/>
+    <circle cx="126" cy="256" r="104" fill="url(#mg-ivory)"/>
+    <circle cx="126" cy="256" r="104" fill="none" stroke="#8a6d1c" stroke-width="1.5" opacity="0.5"/>
+    <text x="126" y="256" text-anchor="middle" dominant-baseline="central"
+          font-family="Georgia, 'Times New Roman', serif" font-size="152" font-weight="600"
+          fill="#21242c">P</text>
+
+    <!-- S tile -->
+    <circle cx="386" cy="256" r="120" fill="url(#mg-gold)"/>
+    <circle cx="386" cy="256" r="104" fill="url(#mg-ivory)"/>
+    <circle cx="386" cy="256" r="104" fill="none" stroke="#8a6d1c" stroke-width="1.5" opacity="0.5"/>
+    <text x="386" y="256" text-anchor="middle" dominant-baseline="central"
+          font-family="Georgia, 'Times New Roman', serif" font-size="152" font-weight="600"
+          fill="#21242c">S</text>
+  </g>
+</svg>

apps/web/public/logo.svg

@@ -0,0 +1,43 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512" width="512" height="512" role="img" aria-label="PubScript">
+  <defs>
+    <radialGradient id="ps-bg" cx="48%" cy="40%" r="78%">
+      <stop offset="0" stop-color="#2b313d"/>
+      <stop offset="1" stop-color="#15181f"/>
+    </radialGradient>
+    <linearGradient id="ps-gold" x1="0" y1="0" x2="0" y2="1">
+      <stop offset="0" stop-color="#f9edc4"/>
+      <stop offset="0.27" stop-color="#e8c76a"/>
+      <stop offset="0.52" stop-color="#c9a23a"/>
+      <stop offset="0.72" stop-color="#f4d683"/>
+      <stop offset="1" stop-color="#937420"/>
+    </linearGradient>
+    <radialGradient id="ps-ivory" cx="45%" cy="38%" r="72%">
+      <stop offset="0" stop-color="#fffdf4"/>
+      <stop offset="0.72" stop-color="#f5ecd6"/>
+      <stop offset="1" stop-color="#e8dcbd"/>
+    </radialGradient>
+    <filter id="ps-shadow" x="-35%" y="-35%" width="170%" height="170%">
+      <feDropShadow dx="0" dy="7" stdDeviation="11" flood-color="#000000" flood-opacity="0.45"/>
+    </filter>
+  </defs>
+
+  <rect width="512" height="512" rx="104" fill="url(#ps-bg)"/>
+
+  <g filter="url(#ps-shadow)">
+    <!-- P tile -->
+    <circle cx="126" cy="256" r="120" fill="url(#ps-gold)"/>
+    <circle cx="126" cy="256" r="104" fill="url(#ps-ivory)"/>
+    <circle cx="126" cy="256" r="104" fill="none" stroke="#8a6d1c" stroke-width="1.5" opacity="0.5"/>
+    <text x="126" y="256" text-anchor="middle" dominant-baseline="central"
+          font-family="Georgia, 'Times New Roman', serif" font-size="152" font-weight="600"
+          fill="#21242c">P</text>
+
+    <!-- S tile -->
+    <circle cx="386" cy="256" r="120" fill="url(#ps-gold)"/>
+    <circle cx="386" cy="256" r="104" fill="url(#ps-ivory)"/>
+    <circle cx="386" cy="256" r="104" fill="none" stroke="#8a6d1c" stroke-width="1.5" opacity="0.5"/>
+    <text x="386" y="256" text-anchor="middle" dominant-baseline="central"
+          font-family="Georgia, 'Times New Roman', serif" font-size="152" font-weight="600"
+          fill="#21242c">S</text>
+  </g>
+</svg>

apps/web/src/components/Topbar.tsx

@@ -29,7 +29,14 @@ export function Topbar({ onOpenPalette, onOpenExport }: TopbarProps) {
 
   return (
     <header className="ps-topbar">
-      <h1 style={{ margin: 0, fontSize: 15, fontWeight: 600 }}>{title}</h1>
+      <img
+        src="/logo-mark.svg"
+        alt="PubScript"
+        className="ps-topbar-logo"
+        style={{ height: 26, width: "auto", marginRight: 12, display: "inline-block", verticalAlign: "middle" }}
+      />
+
+      <h1 style={{ margin: 0, fontSize: 15, fontWeight: 600, display: "inline-flex", verticalAlign: "middle" }}>{title}</h1>
 
       {autoSave && <Badge variant="success">✓ autosave on</Badge>}