fix(lang): add missing Japanese (ja) translation for "Playing in Picture-in-Picture"

[greymoth-jp]

lang/en.json has the string "Playing in Picture-in-Picture", but lang/ja.json was missing it. It was added in #8113 (document picture-in-picture support), which updated en.json and de.json but not the Japanese file, so the string falls back to English for Japanese users.

This adds the Japanese translation, keeping the wording consistent with the existing PiP entries already in ja.json ("Picture-in-Picture" -> "ピクチャーインピクチャー", "Exit Picture-in-Picture" -> "ピクチャーインピクチャー機能の終了"). The meaning matches the German string added in the same PR ("Wird im Bild-im-Bild-Modus wiedergegeben").

After this change ja.json covers every key present in en.json.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.14%. Comparing base (81b3cb4) to head (a4f4db9).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9209      +/-   ##
==========================================
- Coverage   84.78%   84.14%   -0.64%     
==========================================
  Files         120      120              
  Lines        8167     8167              
  Branches     1967     1967              
==========================================
- Hits         6924     6872      -52     
- Misses       1243     1295      +52     

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