feat: add upload preparation progress

[kunal-9090]

Closes #789

Summary

• add visible upload preparation progress feedback to the analysis page
• update progress during file selection, PDF conversion, image optimization, and ready states
• reset progress when the selected upload is removed

Validation

• python -m py_compile app.py
• git diff --check

[github-actions]

Thanks for creating a PR for your Issue! ☺️

We'll review it as soon as possible.
In the meantime, please double-check the file changes and ensure that all commits are accurate.

If there are any unresolved review comments, feel free to resolve them. 🙌🏼

[kunal-9090]

Ready for review. I rechecked this PR: there are no failing code checks or unresolved review comments visible. Please review when possible.

[abhinavkdeval08-design]

Hi @kunal-9090 and @neeru24,

As per GSSoC guidelines, contributors are strictly advised to work on and raise PRs only AFTER an issue has been officially assigned to them.

I authored this issue and have already prepared the implementation locally. Raising unassigned PRs creates merge conflicts and disrupts the workflow.

@neeru24 Could you please officially assign Issue #789 to me so I can raise my PR? Thank you!