Fix desktop icon size and update login link

- Add proper macOS icon padding (80% scale) so icon matches system icon sizes
- Strip glass/glow effects from icon SVG for Liquid Glass compatibility
- Add separate flat layer SVGs for Apple Icon Composer
- Regenerate all raster icon formats (png, icns, ico)
- Update "Learn more" link to https://sandboxagent.dev/

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

foundry/packages/frontend/src/components/mock-onboarding.tsx

@@ -319,7 +319,7 @@ export function MockSignInPage() {
 
         {/* Footer */}
         <a
-          href="https://sandbox-agent.dev"
+          href="https://sandboxagent.dev/"
           target="_blank"
           rel="noopener noreferrer"
           style={{