add Portuguese localisation

[valeriocomo]

Context

Derived from requirement RF-PCE-002. Part of #647.

Scope

Add the Portuguese (pt) UI localisation: translate every string the editor exposes,
using en.json as the reference for the key structure.

Use the generic pt tag rather than a country-specific one. The i18n setup has
nonExplicitSupportedLngs: true, so a browser reporting pt-PT or pt-BR resolves to
pt with no extra configuration. Where the two variants diverge, prefer European
Portuguese, since the requirement targets contributors to the European catalogue.

Affected areas

• src/i18n/locales/pt.json — new file, mirroring the keys of en.json (~23 KB)
• src/i18n/index.ts — register the locale so it appears in the language picker

Non-functional constraints

• RNF-PCE-006 — Long-term sustainability: the translation has to stay complete after
  merge, so it must pass the completeness check introduced in Add new localisations #647.

Acceptance criteria

• pt.json covers every key present in en.json
• The locale is selectable in the language picker
• The translation is reviewed by a Portuguese speaker
• Terminology is consistent with the publiccode.yml specification: field names and
  standard terms keep their meaning rather than being translated ad hoc