papers: add Japanese to cli.mjs LANGS so -ja .tex files build
The build pipeline had "en, da, es, zh" but no "ja" in LANGS, so
existing -ja translations (e.g., complex-ja.tex) were never compiled
even though the .tex source and references in cards point at -ja.pdf.
Add "ja" to LANGS and LANG_NAMES so the build picks up Japanese
sources alongside the others. Papers without a -ja.tex are still
filtered out via the texExists check, so this is safe to add globally.