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/public/favicon.svg

@@ -0,0 +1,12 @@
+<svg width="32" height="32" viewBox="0 0 30 30" xmlns="http://www.w3.org/2000/svg">
+  <defs>
+    <linearGradient id="sp-logo-grad" x1="0" y1="0" x2="1" y2="1">
+      <stop offset="0%" stop-color="#2741ce" />
+      <stop offset="100%" stop-color="#7b9bfa" />
+    </linearGradient>
+  </defs>
+  <rect width="30" height="30" rx="9" fill="url(#sp-logo-grad)" />
+  <path d="M15 7.5 22 11.25 15 15 8 11.25 15 7.5Z" fill="#fff" fill-opacity="0.95" />
+  <path d="M9.6 14.4 15 17.3l5.4-2.9L22 15.25 15 19 8 15.25l1.6-.85Z" fill="#fff" fill-opacity="0.6" />
+  <path d="M9.6 18.4 15 21.3l5.4-2.9L22 19.25 15 23 8 19.25l1.6-.85Z" fill="#fff" fill-opacity="0.32" />
+</svg>