add Spanish localisation

## Context

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

## Scope

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

## Affected areas

- `src/i18n/locales/es.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 #647.

## Acceptance criteria

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

Start by comparing the new src/i18n/locales/es.json with en.json, then inspect src/i18n/index.ts to see how locales are registered. Run the completeness check introduced in #647. Done means every English key has a Spanish translation, the locale appears in the language picker, and terminology has been reviewed by a Spanish speaker.

Written by the indexing model from the issue text.