feat(ui): multi-select bulk delete and restore for documents

[nancysangani]

Closes #422

📝 What does this PR do?

Adds multi-select checkboxes to document items in the sidebar, a
contextual bulk action bar, and bulk restore support in the Trash tab.

New file: frontend/src/components/document/BulkActionBar.tsx
Modified: frontend/src/components/document/DocumentSidebar.tsx

Changes:

• Checkboxes appear on hover (or stay visible when checked) next to
  every document in both the Active and Trash tabs
• Selecting one or more documents shows a BulkActionBar below the
  tab headers with a count, action button, and clear (×) button
• Active tab: bulk action bar shows Delete — calls
  DELETE /api/v1/documents/:id in parallel via Promise.allSettled,
  so partial failures are reported without blocking succeeded deletes
• Trash tab: bulk action bar shows Restore — calls
  POST /api/v1/documents/:id/restore in parallel
• Selection is cleared when switching tabs or after any bulk operation
• Single-item delete and single-item restore buttons are unchanged

🗂️ Type of Change

• ✨ New feature

✅ Self-Review Checklist

• Branch based on dev, not main
• No secrets / API keys added
• No changes to main or HuggingFace deployment config
• Follows existing code style and component patterns
• Accessible — checkboxes have aria-label, action bar has
  role="toolbar" and aria-label
• Uses Promise.allSettled so one failing delete/restore doesn't
  block the rest

[nancysangani]

Hi @param20h please review the PR when you get a chance. Thanks!

[github-actions]

🎉 Congratulations on getting your Pull Request merged! 🎉

Thank you for contributing to PDF-Assistant-RAG as part of GSSoC '26! 🚀

Keep up the great work! ✨