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.

landing/.astro/settings.json

@@ -0,0 +1,5 @@
+{
+	"_variables": {
+		"lastUpdateCheck": 1770647827872
+	}
+}