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>