add Swedish localisation

## Context

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

## Scope

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

## Affected areas

- `src/i18n/locales/sv.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

- [ ] `sv.json` covers every key present in `en.json`
- [ ] The locale is selectable in the language picker
- [ ] The translation is reviewed by a Swedish 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/sv.json with en.json to understand the complete key set, then inspect src/i18n/index.ts for locale registration and the completeness check introduced in #647. The work is done when every English key has a Swedish translation, sv is selectable in the language picker, terminology follows publiccode.yml, and a Swedish speaker has reviewed it.

Written by the indexing model from the issue text.