Changes:
- Drag & drop zones — Replaced plain file inputs with visual drop zones featuring upload/download icons and browse links.
- File selection feedback — Files display with contextual icons, size info, and a remove button (with fade-in animation).
- Processing state — Encrypt/Decrypt buttons show a spinning indicator and status text while working.
- Toast icons — Notifications now include contextual icons for warning, error, and success types.
- Card hover effects — Subtle box-shadow on card hover; logo glow on brand hover.