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/strings_en.g.dart

@@ -1675,15 +1675,6 @@ class Translations$videoControls$en {
 	/// en: 'No subtitles found'
 	String get noSubtitlesFound => 'No subtitles found';
 
-	/// en: 'No subtitles available'
-	String get noSubtitlesAvailable => 'No subtitles available';
-
-	/// en: 'No audio tracks available'
-	String get noAudioTracksAvailable => 'No audio tracks available';
-
-	/// en: 'No tracks available'
-	String get noTracksAvailable => 'No tracks available';
-
 	/// en: 'Subtitle downloaded'
 	String get subtitleDownloaded => 'Subtitle downloaded';
 
@@ -5724,9 +5715,6 @@ extension on Translations {
 		} ?? switch (path) {
 			'videoControls.language' => 'Language',
 			'videoControls.noSubtitlesFound' => 'No subtitles found',
-			'videoControls.noSubtitlesAvailable' => 'No subtitles available',
-			'videoControls.noAudioTracksAvailable' => 'No audio tracks available',
-			'videoControls.noTracksAvailable' => 'No tracks available',
 			'videoControls.subtitleDownloaded' => 'Subtitle downloaded',
 			'videoControls.subtitleDownloadedNotApplied' => 'Subtitle downloaded, but it could not be selected',
 			'videoControls.subtitleDownloadFailed' => 'Failed to download subtitle',
@@ -6234,11 +6222,11 @@ extension on Translations {
 			'watchTogether.syncing' => 'Syncing...',
 			'watchTogether.joinWatchSession' => 'Join Watch Session',
 			'watchTogether.enterCodeHint' => 'Enter 5-character code',
-			_ => null,
-		} ?? switch (path) {
 			'watchTogether.pasteFromClipboard' => 'Paste from clipboard',
 			'watchTogether.pleaseEnterCode' => 'Please enter a session code',
 			'watchTogether.codeMustBe5Chars' => 'Session code must be 5 characters',
+			_ => null,
+		} ?? switch (path) {
 			'watchTogether.joinInstructions' => 'Enter the host\'s session code to join.',
 			'watchTogether.failedToCreate' => 'Failed to create session',
 			'watchTogether.failedToJoin' => 'Failed to join session',