#502 i18n: delete three orphan translation keys

[Anonymous]

Originally created by: Redo-San

These keys have zero call sites outside Style/lang/. Verified with a repo-wide search excluding Style/lang/, node_modules/.

Key	Present in	Evidence
md.download_json	ar, de, es, fr, ja, ko, zh	0 references outside Style/lang/
test.translate	de, fr, zh	0 references outside Style/lang/
simple.cert_desc	ar, de, es, fr, zh	code builds simple.cert_desc_ + certType at Style/simplified_renderers.js:851; the exact key is never read

Do NOT touch

simple.ph_* and simple.c2pa_* are real keys (read at Style/simplified_renderers.js:111,118,125,...).

Acceptance criteria

• [ ] rg -n "md\.download_json|test\.translate" --glob "!Style/lang/**" → 0 hits.
• [ ] After node scripts/sync-i18n-json-to-js.js, all 8 .js files report 1062 keys (today: en=1062, ar=1078, de/es/fr/zh=1066, ja/ko=1064).