refactor(flottes): redesign document manager and dialogs
Summary
- Migrate fleet file manager to shadcn with improved doc type color badges (clearer green for medical certificates, blue for authorizations)
- Redesign add-folder dialog to shadcn Dialog with Tailwind styling
- Redesign upload-file dialog to shadcn with file type selector and drag-drop area
- Major cleanup: reduce fleet-file-manager from ~1300 to ~700 lines
Closes VSN-2504