feat(ui): implement dual-ui architecture and dynamic slash menus

- Introduced the new `ModernUI` component alongside the refactored `ClassicUI`.
- Updated `App.tsx` to handle dynamic toggling between `classic` and `modern` interfaces via `kodaSettings.uiMode`.
- Added the Interface Style selector (Classic CLI vs Modern Pro) to `KodaSettingsTab.tsx`.
- Restored and styled the dynamic interaction menus (commands `/` and files `@`) to flawlessly anchor above the chat input, matching the TitleBar aesthetic.
- Imported modern typography (Inter, Outfit) into `index.css`.
- Updated `KodaSettings` type definitions to persist the chosen interface mode.

Author: antojunimaia-ui