[FE-55] Add bulk actions to asset list — multi-select with bulk status update, assign, and delete

[yusuftomilola]

Overview

The asset list page shows up to hundreds of assets but only supports individual actions per row. Operations teams need to select multiple assets and apply a change to all of them in one action — e.g., change a lab full of laptops to MAINTENANCE status before a repair event. Depends on BE-68 for the bulk API endpoints.

Context

• Asset list page: frontend/app/(dashboard)/assets/page.tsx
• Bulk API endpoints (from BE-68): PATCH /assets/bulk/status, PATCH /assets/bulk/assign, DELETE /assets/bulk
• The table is rendered using a data table component — extend it with a checkbox column

Acceptance Criteria

• Add a checkbox column to the asset table — checking any row enters "selection mode"
• "Select All" checkbox in the header selects all rows on the current page
• A floating action bar appears at the bottom of the screen when 1+ assets are selected, showing: selected count, "Update Status", "Assign", "Delete" buttons
• "Update Status" opens a dropdown to pick the new status, then calls PATCH /assets/bulk/status
• "Assign" opens a modal with user selector and department selector, then calls PATCH /assets/bulk/assign
• "Delete" shows a confirmation dialog with the count, then calls DELETE /assets/bulk (ADMIN only, hide for other roles)
• On completion: show a result toast — "Updated 12 assets. 2 skipped (already retired)."