Add darker logotype variant for light mode

The existing gradient logo (#c4dede-#495669) was designed for dark
backgrounds. Adds logotype-light.svg with the same paths but a
deeper gradient (#2a6b6b-#1c2a3a) for better contrast on white,
and swaps the src based on the current theme.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/arc-web/app/layouts/app-shell.tsx

@@ -69,7 +69,7 @@ export default function AppShell() {
             <div className="flex items-center">
               <div className="shrink-0">
                 <Link to="/start">
-                  <img alt="Arc" src="/logotype.svg" className="h-8 w-auto" />
+                  <img alt="Arc" src={theme === "dark" ? "/logotype.svg" : "/logotype-light.svg"} className="h-8 w-auto" />
                 </Link>
               </div>
               <div className="hidden md:block">