fix(i18n): drop unused video-control strings and restore spinner lookups

`clean_translations.py --strict` reported seven unused keys.

Three are genuinely dead: TrackSelectionHelper.getEmptyMessage was
removed as unreachable in 4307c49c, and the sheets that need an empty
state carry their own strings. Removes them from every locale.

The four accessibility keys are false positives. tv_number_spinner
aliased the subtree as `final a11y = ...accessibility`, which the static
scanner cannot follow — its docstring says as much. Binds the documented
`final t = Translations.of(context)` instead and hoists the two labels,
so the semantics stay identical and the scanner sees the chains.

lib/i18n/ko.i18n.json

@@ -546,9 +546,6 @@
     "searchSubtitles": "자막 검색",
     "language": "언어",
     "noSubtitlesFound": "자막을 찾을 수 없습니다",
-    "noSubtitlesAvailable": "사용 가능한 자막 없음",
-    "noAudioTracksAvailable": "사용 가능한 오디오 트랙 없음",
-    "noTracksAvailable": "사용 가능한 트랙 없음",
     "subtitleDownloaded": "자막이 다운로드되었습니다",
     "subtitleDownloadedNotApplied": "자막을 다운로드했지만 선택할 수 없습니다",
     "subtitleDownloadFailed": "자막 다운로드에 실패했습니다",