Add Spanish (es) translation

[erdanieee]

Adds Spanish (es) as a new interface language.

Two files, matching how fr and de were added:

File	What it is
po/es.po	Source catalogue — 2418 of 2418 messages of the current template
locale/es/LC_MESSAGES/boxes.py.mo	Compiled catalogue

No code changes are needed. Language discovery is entirely glob based —
BServer.getLanguages() globs locale/*/LC_MESSAGES/boxes.py.mo,
setup.py globs po/*.po, MANIFEST.in already covers po/*, and
BServer.lang_re already matches both es and es-ES, so
Accept-Language: es-ES resolves to this catalogue.

The compiled .mo is committed because getLanguages() looks for .mo
files, not .po files — without it the language never appears in the
selector. This is also why the committed catalogue matters in the Docker
image: setup.py shells out with os.system(f"msgfmt {po} -o ...")
without checking the return code, so on a machine without gettext the
build silently produces no catalogue at all.

Translation decisions

• Technical identifiers are left unchanged: single-letter edge codes
  (e, F, Š, …), short section parameters (x, y, h, sx, sh, …),
  output formats (svg, dxf, lbrn2, …), pulley profiles (GT2_2mm, MXL, …)
  and pressure units (Pa, mmHg, …).
• Only <option> labels are translated, never the values. arg2html
  emits <option value="{e}">{_(e)}</option>, so form submission and the
  CLI keep working with the original English values.
• Where a parameter name and its own help text describe different
  quantities, the label follows the help, so the two agree on screen.
  Example: --min_strength is documented as "strength of smallest wrench
  (in mm)", which is a thickness, so it is labelled grosor mínimo rather
  than a literal resistencia mínima.
• Neutral Spanish, impersonal/infinitive register for actions
  (Generar, Descargar, Guardar en la URL).

Notes for review

• The catalogue is built against the committed template
  (POT-Creation-Date: 2026-03-28). Generators added after that date —
  FrameLoom — are not covered yet and will be picked up the next time
  po/boxes.py.pot is regenerated.
• The .mo was compiled with pybabel compile rather than msgfmt, so its
  header carries Generated-By: Babel 2.10.3. Regenerating it with msgfmt
  produces a functionally identical but byte-different file.
• The CLI does not translate argparse help for any language (de, fr,
  zh_CN behave the same), so nothing changes there. get_translation()
  does resolve correctly, so LANGUAGE=es reaches box.translations, which
  is what UBox uses for its engraved part labels.

Verification

• /?language=es and /TypeTray?language=es render in Spanish; the language
  selector lists es and marks it as selected (the directory name and the
  catalogue's Language: header match, which is what that comparison relies on).
• Generating an SVG through the form still works, confirming that translating
  choice labels did not affect submitted values.
• The error page substitutes correctly: "Error al generar Caja universal".
• pytest — 194 passed, 9 skipped, including the byte-for-byte
  examples/*.svg comparison; the suite also passes with LANGUAGE=es set,
  so no translated string leaks into rendered output.
• pre-commit run --files po/es.po locale/es/LC_MESSAGES/boxes.py.mo — clean.
• A consistency pass over the catalogue checks that every %s/%d, URL,
  image path, HTML tag and entity is preserved, and that no entry is marked
  fuzzy.