0.36.1: app logo as browser-tab favicon

Add frontend/public/favicon.svg (the TopNav LogoMark glyph as a
standalone SVG) and link it from index.html so the StackPilot logo
shows in browser tabs. Vite copies public/ into dist on build.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/index.html

@@ -3,6 +3,7 @@
   <head>
     <meta charset="UTF-8" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
+    <link rel="icon" type="image/svg+xml" href="/favicon.svg" />
     <title>StackPilot</title>
   </head>
   <body>