feat: replace native confirm() with styled delete confirmation modal on Knowledge Base page

[androemeda]

Closes #48

Summary

Replaces the native browser confirm() dialog on the /admin/knowledge-base page with a styled in-app modal that is consistent with the rest of the UI. The modal displays the filename, a warning that the action cannot be undone, and Cancel / Delete buttons.

Explanation of Implementation

• Added two new state variables — showDeleteModal (boolean) and fileToDelete (string | null) — to control the modal.
• Refactored handleDeleteFile to only set state (open the modal) instead of calling confirm() and making the API call directly.
• Extracted the API call logic into a new confirmDelete function, which is called when the user clicks Delete in the modal.
• Added the delete confirmation modal JSX inline in the same file, following the exact same pattern already used by the Upload Modal and File Preview Modal on the same page — no new component file needed.
• Styling uses the existing site palette: bg-red-100 / text-red-600 for the icon badge, bg-red-600 for the Delete button, and border-slate-300 for Cancel — consistent with the blue-600 / slate / red-600 palette used throughout the app.

Screenshots

Before

After

Testing

• Navigate to /admin/knowledge-base and click the trash icon — confirm the new modal appears (not a browser alert)
• Click Cancel — confirm the modal closes and no file is deleted
• Click Delete — confirm the file is removed from the list and the modal closes