i18n: replace hardcoded chat action labels with translation keys

[Senzo13]

Context

Several chat action labels and tooltips are still hardcoded in French inside frontend templates. This makes the UI feel inconsistent when the user switches language.

Examples found in:

• web/static/js/chat.js
• web/static/js/generation.js
• web/static/js/db.js

Look for labels such as Copier, Lire, Editer, Regenerer, Télécharger, Original, Modifié, and similar title attributes.

Suggested approach

Replace hardcoded user-facing strings with the existing translation helper pattern, then add missing keys in web/static/js/i18n.js for at least French and English.

Acceptance criteria

• Chat action buttons and common tooltips use i18n keys instead of inline French text.
• French and English translations are present.
• Existing behavior and icons do not change.
• node --check web/static/js/chat.js web/static/js/generation.js web/static/js/db.js web/static/js/i18n.js passes.