NiceVPN R12

- Fixed the broken admin login page with scoped login-only CSS.
- Added per-platform application name (software_platforms.app_name).
- Removed the global application-name input from download-page settings.
- The detected platform's app name is now used in the download Hero.
- Improved mobile download buttons without changing the two-column card layout.
- Button titles remain single-line and receive more usable horizontal space.
- Existing download page structure, Hero, cards, colors and actions remain intact.