Add Slovak (sk) translation + fix missing strings across all locales

[pipozzz]

What's new

Slovak language support

Added complete Slovak (slovenčina) translation across the entire platform:

• Backend: backend/lang/sk.json — 664 keys fully translated
• Frontend: frontend/src/locales/sk.po — 2330 entries, 0 missing
• Config: Added sk to Locale.php enum, lingui.config.ts, locales.ts, dateLocales.ts, and LanguageSwitcher
• README: Added README.sk.md and linked it from all other translated READMEs

Fixed missing translations in all other locales

Filled in all missing msgstr entries for: de, fr, nl, hu, pt, pt-br, vi, tr, se, zh-cn, zh-hk, es, it, pl

Before:

• de/fr/nl/hu/pt/pt-br/vi/tr/se/zh-cn/zh-hk — 33 missing each
• pl — 121 missing
• es/it — 3 missing each

After:

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

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Files changed

• backend/app/Locale.php — added SK enum case
• backend/lang/sk.json — new file (664 keys)
• frontend/lingui.config.ts — added "sk" locale
• frontend/src/locales.ts — added sk to type, array, flag map, name map
• frontend/src/locales/sk.po — new file
• frontend/src/locales/sk.js — compiled catalog
• frontend/src/utilites/dateLocales.ts — added sk dayjs import + format
• frontend/src/components/common/LanguageSwitcher/index.tsx — added case "sk"
• frontend/src/locales/{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.sk.md — new file
• README.md + all other README.*.md — added Slovak 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
• Slovak locale selectable in language switcher with 🇸🇰 flag

[github-actions]

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[pipozzz]

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