Add Japanese (ja) exercise instructions

[Jada-Q]

Summary

Adds Japanese (ja) exercise instructions across the dataset.

• Adds instructions.ja for all 1,324 exercises
• Adds instruction_steps.ja for all 1,324 exercises
• Updates README language counts and examples from 9 to 10 languages
• Adds Japanese to the exercise browser language tabs
• Adds instructions_ja to SQL schemas and generated INSERT exports in setup.html and the browser DB setup modal

Translation note

The Japanese text was machine-translated from the English source instructions (Claude), using standard Japanese gym/fitness vocabulary (katakana equipment terms, です・ます instruction style). It was then spot-reviewed by me — I've lived in Japan for 18 years and read Japanese daily — across a stratified sample covering all 10 body-part categories.

Validation

• All 1,324 exercises have both instructions.ja and instruction_steps.ja
• Japanese step count matches English step count for every exercise
• No empty steps, no untranslated English residue (automated check)
• data/exercises.json parses and conforms to the existing schema (additionalProperties allows the new language key, following the hi/pl/ko pattern)
• Browser verified: 日本語 tab renders correctly in index.html

🤖 Generated with Claude Code