feat: use app icon as sidebar logo

Replace the Sparkles icon gradient placeholder with the actual app
icon from assets/icons/icon.png. Import it as a Vite asset so it
works in both dev and production builds.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/renderer/assets.d.ts

@@ -0,0 +1,4 @@
+declare module '*.png' {
+  const src: string;
+  export default src;
+}