i18n: add Portuguese (pt) UI translation catalog

[Kayforkind]

Problem

The UI ships in English, Spanish, German, and French. Portuguese is the next most-requested language family and the infra already supports adding one.

Scope

• Follow the existing pattern exactly: src/locales/pt/translation.json mirroring the key structure of src/locales/en/translation.json, plus the import and LANGUAGES entry in src/i18n.ts.
• Translate naturally (pt-PT or pt-BR, say which in the PR). If you're not a native speaker, please say so — machine-translated catalogs will be reviewed, not merged blind.

Acceptance criteria

• All keys present — the i18n token-parity tests (src/lib/i18n.test.ts) pass.
• npm test green; no other source files touched.
• Portuguese appears in the in-app language switcher.