chore: Polish localization for STR_SHOW_HIDDEN_FILES (#1490)

## Summary

Polish localization for `STR_SHOW_HIDDEN_FILES`

---

### AI Usage

While CrossPoint doesn't have restrictions on AI tools in contributing,
please be transparent about their usage as it
helps set the right context for reviewers.

Did you use AI tools to help write this code? _**NO**_

lib/I18n/translations/polish.yaml

@@ -81,6 +81,7 @@ STR_SCREEN_MARGIN: "Marginesy ekranu"
 STR_PARA_ALIGNMENT: "Wyjustowanie paragrafów"
 STR_HYPHENATION: "Dzielenie wyrazów"
 STR_TIME_TO_SLEEP: "Czas do uśpienia"
+STR_SHOW_HIDDEN_FILES: "Pokaż ukryte pliki"
 STR_REFRESH_FREQ: "Częstotliwość odświeżania"
 STR_KOREADER_SYNC: "KOReader Sync"
 STR_CHECK_UPDATES: "Sprawdź uaktualnia"