Add dataset validator

[yazmorukyaz]

Summary

Adds a dependency-free Node validator for data/exercises.json and the media folders.

The validator checks:

• JSON parseability and array shape
• required exercise fields
• four-digit unique IDs
• six-language instructions and instruction_steps coverage
• secondary_muscles array shape
• media path format derived from id and media_id
• exact Gym visual attribution
• missing and orphan image/GIF files
• duplicate exercise names as warnings, not failures

Validation

• node --check scripts/validate-dataset.mjs
• node scripts/validate-dataset.mjs

Result:

• 1,324 records
• 1,324 images
• 1,324 GIFs
• 0 missing required fields
• 0 missing language instructions
• 0 duplicate IDs
• 0 missing media files
• 0 orphan media files
• 6 duplicate-name warnings