Add Spanish signage translation

[Commons-Lab-1]

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

1. Copy signage-en.svg to signage-es.svg
2. Translate all text strings
3. Key phrases: "PLAIN WATER ONLY" → "SOLO AGUA", "1 bottle (~1L) maximum" → "1 botella (~1L) máximo"
4. Keep layout identical

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