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/global.d.ts

@@ -38,3 +38,6 @@ declare module "*.bmp" {
   const content: string;
   export default content;
 }
+
+// Global build-time constants
+declare const __APP_VERSION__: string;