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/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>}