feat(ui): enhance drag-and-drop upload feedback with animated icon an…

[NaitikVerma6776]

📋 PR Checklist

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🔗 Related Issue

Closes #557

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📝 What does this PR do?

The core drag-and-drop highlight feature was already implemented in
DocumentSidebar.tsx via react-dropzone's isDragActive state
(border highlight, background change, "Drop files here" message in 4 languages,
auto-reset on drag leave/drop).

This PR adds extra visual and accessibility polish on top of the existing feature:

Improvements:

• Icon swaps to an animated bouncing UploadCloud icon during drag (vs static Upload icon)
• "Drop files here" text turns primary color + bold during drag for stronger visual feedback
• Added aria-live="polite" so screen readers announce the drop message change

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🗂️ Type of Change

• 🐛 Bug fix
• ✨ New feature
• 🔧 Refactor / code cleanup
• 📝 Documentation update
• 🎨 UI / styling change
• ⚙️ CI / tooling / config change
• 🧪 Tests

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🧪 How was this tested?

• Ran the frontend locally (npm run dev inside frontend/)
• Ran the backend locally (uvicorn app.main:app --reload)
• Tested the affected API endpoints manually
• Added / updated tests

Verified npm run lint (0 errors) and npm run build pass successfully.

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📸 Screenshots (if UI change)

N/A — toolbar UI improvement, visible when dragging a file over the upload area

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⚠️ Anything to flag for reviewers?

• Core drag-and-drop highlight/message functionality already existed and is untouched
• Only additive styling and accessibility improvements — no changes to onDrop, validation, or upload logic
• Existing upload functionality continues to work correctly

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✅ Self-Review Checklist

• My branch is based on dev, not main
• I have not added any secrets / API keys
• I have not modified main branch or any HuggingFace deployment config
• My code follows the existing style (no unnecessary formatting changes)
• I have updated relevant docs / comments if needed

[NaitikVerma6776]

Note: The "Run backend pytest suite with coverage" failure is pre-existing on dev —backend/app/services/layout_parser.py imports pymupdf4llm but it's missing from requirements.txt (only PyMuPDF is listed, which is a different package). This is unrelated to this PR, which only modifies frontend/src/components/document/DocumentSidebar.tsx.

I've opened a separate PR to fix this dependency issue #601. so kindly merge this issue and assign the upcoming issue to resolve unnecessary fails.