Add French signage translation

[Commons-Lab-1]

Translate mechanical/signage/signage-en.svg to French.

1. Copy signage-en.svg to signage-fr.svg
2. Translate all text strings
3. Key phrases: "PLAIN WATER ONLY" → "EAU CLAIRE UNIQUEMENT", "1 bottle (~1L) maximum" → "1 bouteille (~1L) maximum"
4. Keep layout identical

See mechanical/signage/README.md for content guidelines.