add Finnish localisation

## Context

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

## Scope

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

## Affected areas

- `src/i18n/locales/fi.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

- [ ] `fi.json` covers every key present in `en.json`
- [ ] The locale is selectable in the language picker
- [ ] The translation is reviewed by a Finnish speaker
- [ ] Terminology is consistent with the `publiccode.yml` specification: field names and
standard terms keep their meaning rather than being translated ad hoc
- [ ] Compound words and the longer strings Finnish tends to produce do not break the
form layout

Use src/i18n/locales/en.json as the reference, then create src/i18n/locales/fi.json with matching keys and register it in src/i18n/index.ts. Run the completeness check introduced in #647, verify the locale appears in the language picker, and have a Finnish speaker review terminology and layout for longer strings.

Written by the indexing model from the issue text.