feat(ui): full-page drag-and-drop upload overlay with hover animations

[nancysangani]

Closes #417

📝 What does this PR do?

Adds a full-page drag-and-drop upload zone that activates whenever the
user drags accepted files anywhere over the dashboard window — not just
over the small upload box in the sidebar.

New files:

• frontend/src/hooks/useDashboardDrop.ts — custom hook that attaches
  window-level dragenter / dragleave / drop listeners, uses a
  ref-based counter to avoid flickering from nested elements, and filters
  files by extension before calling onDrop
• frontend/src/components/document/DashboardDropOverlay.tsx — full-
  screen overlay with blurred backdrop, animated dashed border card,
  pulsing icon ring, and bouncing upload icon

Modified:

• frontend/src/app/dashboard/page.tsx — wires up the hook and renders
  the overlay; dropped files are uploaded via the existing
  api.postForm pipeline and the document list refreshes on completion

Animations:

• Backdrop: fade-in on mount
• Card: zoom-in-95 fade-in on mount
• Icon ring: animate-ping pulse + animate-bounce icon
• All powered by Tailwind animate-in — no extra dependencies

Behaviour:

• Only activates for accepted MIME types (PDF, DOCX, TXT, MD)
• Drag counter ref prevents flicker when cursor moves over child elements
• Overlay is disabled (disabled prop) when user is not authenticated
• Multiple files dropped simultaneously are all uploaded sequentially
• Falls back gracefully if any individual file fails (logs error,
  continues with remaining files)
• Existing sidebar dropzone is unchanged — both work independently

🗂️ 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
• No new dependencies — uses only existing Tailwind utilities and
  React built-ins
• Accessible — overlay has role="region", aria-label, and
  aria-live="assertive"

[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! ✨