[ja.json] Missing translation for Polaris.ResourceList.showing

[Armeria-Nakano]

Summary

This is actually a missing translation (bug) in ja.json.

In the Japanese locale, the key Polaris.ResourceList.showing is missing,
so the UI still shows the English text "Showing X {resourceName}" even when using the Japanese locale.

Suggested translation:
"{itemsCount}件の{resourceName}を表示中"

Rationale

Without this translation, Japanese users see a mixed-language UI,
which can cause confusion or make the product feel unfinished.

Adding this key will improve localization consistency across Polaris components.