feat: improve UX with error handling, add README and demo seed data

[roninscript30]

🎭 Summary

This PR enhances the overall user experience and demo readiness of the system by adding:

• Error state UI for upload failures (Add error-state UI for permanent chunk / upload failures #19)
• Comprehensive README documentation (Add README with setup/run instructions (required deliverable) #17)
• Seed data script for demo environment (Add mock data / seed script for demo-ready environment #18)

These improvements ensure better usability, clearer system understanding, and a more impactful demo experience.

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✅ Changes

❗ Error State UI (#19)

• Added user-visible error messages for upload failures
• Displayed clear failure reasons (network, auth, size, etc.)
• Implemented retry mechanism for failed uploads
• Added proper UI feedback (toast/banner/inline states)
• Improved handling of partial uploads and failure scenarios

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📘 README Documentation (#17)

• Added complete project overview and architecture
• Documented tech stack (React, FastAPI, MongoDB, AWS S3)
• Included setup instructions for all services
• Added environment variable reference
• Provided run commands for frontend and backend
• Included demo walkthrough steps

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🎭 Seed Data (#18)

• Implemented seed script for demo-ready environment
• Created sample users (doctor and patient roles)
• Added sample upload history records
• Enabled one-command execution (npm run seed / script)

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🧪 Testing

Error UI

• Simulated network failure → error message displayed
• Simulated backend error → retry option available
• Verified user feedback is clear and actionable

README

• Followed setup steps from scratch → system runs successfully

Seed Data

• Ran seed script → dashboard populated with demo data
• Verified users and upload history present

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🎯 Impact

• Improves user experience during failure scenarios
• Makes system easier to understand and run
• Enhances demo presentation with realistic data
• Aligns with deliverable requirements

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Closes #19
Closes #17
Closes #18