fix(ui): Implement pre-flight upload validation to prevent server crashes

[chinmayy777]

Description

Resolves #31

📦 Stacked PR Notice: > Note for Reviewers: This PR was branched off of my infrastructure fixes in PR #8 so that I could successfully build and test the Docker containers locally.

Currently, the GitHub diff shows additional files changed, but the only new changes for this specific feature are in apps/frontend/src/app/_home/uploadHandlers.ts. Once PR #8 is merged into main, GitHub will automatically update this diff to only show the relevant optimization!

While testing the UI with diverse datasets, I noticed that if a user accidentally uploads a directory lacking valid .dcm or BIDS files, the frontend still fires a heavy, empty POST request. This causes an unhandled 500 Internal Server Error on the FastAPI backend and presents the user with an unhelpful AxiosError toast.

This PR introduces strict Pre-flight Validation in uploadHandlers.ts.

• The UI now fails fast, intercepting empty/invalid directory uploads before hitting the network.
• Users receive immediate, context-aware Toast notifications (e.g., "No valid DICOM files found in the selected folder").
• This saves unnecessary server bandwidth and drastically improves the UX for researchers handling messy datasets.

Checklist

• Code follows style guidelines
• Tests are included and passing (UI manually tested)
• Documentation is updated
• No breaking changes
• Error handling is appropriate (Replaced generic Axios errors with specific pre-flight toasts)
• Performance considerations addressed (Prevents unnecessary API calls)
• Security implications considered

Before the fix:

After the fix: