fix(frontend): prevent invalid file uploads from triggering API calls

[sanchit955]

Fix: Prevent unnecessary API calls for invalid file uploads

Previously, selecting an unsupported or invalid file still triggered an API request to /projects/upload, which resulted in a backend 400 error. This caused unnecessary network calls and a poor user experience.

Changes

• Added client-side file validation (extension + empty file check)
• Prevents API calls for invalid files
• Displays errors using toast notifications instead of relying on backend responses
• Added accept attribute to file input for better UX
• Added defensive validation before form submission

Supported formats

.csv, .xlsx, .json, .parquet, .tsv

Impact

• Improves UX by validating inputs early
• Reduces unnecessary backend load
• Keeps implementation minimal and consistent with existing codebase
• Aligns frontend behavior with supported dataset formats

[sanchit955]

Hi maintainers, I’ve squashed the commits into a single commit and all checks are now passing. Would love your review 🙌