Review the French translation catalogue as a native speaker

[ralleur]

Goal

Review the French interface catalogue as a native speaker so the first public beta does not ship technically correct but unnatural wording.

Scope

• Review app/messages/fr.json against app/messages/en.json and app/messages/de.json.
• Cover setup, room/device management, connection errors and normal panel/phone navigation.
• Preserve every key, placeholder and formatting marker exactly.
• Prefer natural product language over literal translation.
• Do not change product behaviour or add new messages in this issue.

Acceptance criteria

• Every French key has been reviewed in context.
• Placeholders and message structure still match the source catalogues.
• Terminology for Home Assistant, rooms, devices, setup and actions is consistent.
• cd app && npm test passes.
• cd app && npm run check passes.
• The French locale is smoke-tested in the app with no visibly truncated setup labels.

A short note explaining any terminology choice that is not obvious would help future contributors.