feat(frontend): enhance API credentials system and build configuration

Add real API support in demo mode with credential checking, implement build-time version injection from package.json, and refactor update checking with 24-hour caching. Migrate landing page from Vue to Astro with comprehensive UI components including Hero, Features, Benefits, and Tech Stack sections. Update CI/CD workflow with expanded cache paths and security scanner version pinned.

frontend/src/components/ui/BookmarkModal.tsx

@@ -55,7 +55,7 @@ export const BookmarkModal = (props: BookmarkModalProps) => {
     <>
       {/* Backdrop */}
       {props.isOpen && (
-        <div class="fixed inset-0 bg-black/50 z-40" onClick={props.onClose} />
+        <div class="fixed inset-0 bg-black/50 z-40 mt-0" onClick={props.onClose} />
       )}
 
       {/* Modal */}