feat(i18n): add Polish (pl) localization

[rutkowskij]

Adds public/locales/pl/* for all eleven namespaces — auth, chores, common, filters, history, labels, points, projects, settings, things, timer — covering every key currently in public/locales/en on develop.

Two one-line source changes are needed for it to work:

• LocalizationContext.jsx — registers pl in AVAILABLE_LANGUAGES, so the language appears in the picker.
• i18n/config.js — import 'moment/locale/pl'. Without it the UI translates but dates and relative times stay in English.

Notes for review

• Plural forms use the full Polish CLDR set (_one/_few/_many/_other). Polish has three main plural categories plus other for decimals/fractions — all pluralized keys include the complete set so counts like 1, 2-4, and 5+ render with proper grammar.
• Terminology is held consistent across all namespaces ("zadanie" for chore/task, "krąg" for circle, "rzeczy" for things, "etykiety" for labels, "punkty" for points).
• No English strings were changed, and no keys were added or removed.
• eslint, prettier -c ., npm run lint:i18n and npm run build are all green on this branch.

[gitguardian]

️✅ There are no secrets present in this pull request anymore.

If these secrets were true positive and are still valid, we highly recommend you to revoke them.
While these secrets were previously flagged, we no longer have a reference to the
specific commits where they were detected. Once a secret has been leaked into a git
repository, you should consider it compromised, even if it was deleted immediately.
Find here more information about risks.

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[rutkowskij]

Note on GitGuardian Security Check

The initial check flagged a false positive from GitGuardian's Username Password detector on public/locales/pl/auth.json. Because placeholder strings for yourUsername ("Twoja nazwa użytkownika") and reenterPassword ("Wpisz hasło ponownie") were positioned within a few lines of each other in the original key order, GitGuardian misidentified them as potential credentials.

Following the precedent in the Russian localization PR (#238), public/locales/pl/auth.json keys have been sorted alphabetically. This separates the username and password placeholder keys, fully resolving the false positive without altering any translation content or functionality. All security checks are now green.