fix(i18n): localize the new session form

[devswha]

Summary

• move every new-session label, placeholder, progress state, and fallback error into the sidebar namespace
• provide the complete key set for all 10 supported locales
• register the previously omitted French resources so the supported-language selection is effective
• add locale completeness and English/Korean no-mixing render coverage

Verification

• new-session localization tests: 2/2
• all 10 sidebar locale JSON files parse successfully
• npm run typecheck
• npm run lint
• npm run build

[devswha]

Actual browser QA (2026-07-22): the new-session form rendered Session name, Working folder, Cancel, and Create entirely in English; after switching to Korean it rendered 세션 이름, 작업 폴더, 취소, and 만들기 without mixed-language form copy.