Add Finnish translation

[niilorm]

What's new

Finnish language support

Added complete Finnish translation across the entire platform following PR #1210:

• Backend: backend/lang/fi.json — fully translated
• Frontend: frontend/src/locales/fi.po — extracted and fully translated
• Config: Added fi to Locale.php enum, lingui.config.ts, locales.ts, dateLocales.ts, and LanguageSwitcher
• README: Added README.fi.md and linked it from all other translated READMEs

Filled in all resulting msgstr entries for Finnish for other languages.

After:

• All locales — 0 missing ✅ (except ru which was already untranslated)

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Files changed

• backend/app/Locale.php — added FI enum case
• backend/lang/fi.json — new file
• frontend/lingui.config.ts — added "fi" locale
• frontend/src/locales.ts — added fi to type, array, flag map, name map
• frontend/src/locales/fi.po — new file
• frontend/src/locales/fi.js — compiled catalog
• frontend/src/utilites/dateLocales.ts — added fi dayjs import + format
• frontend/src/components/common/LanguageSwitcher/index.tsx — added case "fi"
• frontend/src/locales/{sk,de,fr,nl,hu,pt,pt-br,vi,tr,se,zh-cn,zh-hk,es,it,pl}.po — filled missing translations
• frontend/src/locales/*.js — recompiled catalogs
• README.fi.md — new file
• README.md + all other README.*.md — added Finnish link to language list

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Testing

• yarn messages:extract shows 0 missing for all locales (except ru)
• yarn messages:compile completes successfully
• Finnish locale selectable in language switcher with 🇫🇮 flag

Checklist

• I have read the contributing guidelines.
• My code follows the coding standards of the project.
• I have tested my changes, and they work as expected.
• I understand that this PR will be closed if I do not follow the contributor guidelines and if this PR template is left unedited.

[github-actions]

All contributors have signed the CLA ✍️ ✅
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[daveearley]

Thank you very much for this contribution!