fix: pl translation quality and consistency

[wlzukowski]

Related to #1261

What was done

I reviewed and improved the Polish translation for better consistency, readability, and a more natural flow.

As noted in the issue, translating from .po/.json files alone is context-unaware and can hurt quality, so I went through the app interface directly (creating attendees, events, etc.) rather than translating file-by-file.

What I changed

• Made some consistent translation decisions - e.g. always translating "check-in" as "weryfikacja" (and, consequently, "lista weryfikacyjna") and applied them uniformly across every occurrence.
• Went through the whole app interface (creating attendees, events, etc.) and verified translations in context, adjusting wording to fit how the text actually appears in the UI.
• Adjusted date and time formats to Polish standards.

Files changed

• frontend/src/locales/pl.po - translation updates
• frontend/src/locales/pl.js - recompiled from pl.po (yarn messages:compile)
• frontend/src/utilites/dateLocales.ts - Polish date/time formats
• backend/lang/pl.json - backend string updates

Verification

• Recompiled catalogs with yarn messages:compile
• npx tsc --noEmit - no new errors introduced

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[wlzukowski]

I have read the CLA Document and I hereby sign the CLA