feat: add admin login page and authentication API

- Created a new login page for admin access at frontend/admin/login/index.html.
- Implemented form submission with error handling for login credentials.
- Updated frontend/index.html to mute the video player and removed unnecessary Alpine.js data binding.
- Enhanced manifest URL resolution in frontend/src/main.ts to include cache-busting parameters.
- Modified vite.config.ts to include the new admin login page in the build process.
- Updated nginx.conf to handle admin login routing, authentication, and API proxying for admin actions.

.gitignore

@@ -82,7 +82,7 @@ grafana/
 # Agent / Tool files
 .github/instructions/TODO.md
 .github/instructions/DONE.md
-# (keep AGENTS.md and other instruction files)
+# (keep AGENTS.md, MEMORY.md, and other instruction files)
 
 # Temporary architecture artifacts
 **/*.tmp