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_zh.g.dart

@@ -704,9 +704,6 @@ class Translations$videoControls$zh extends Translations$videoControls$en {
 	@override String get searchSubtitles => '搜索字幕';
 	@override String get language => '语言';
 	@override String get noSubtitlesFound => '未找到字幕';
-	@override String get noSubtitlesAvailable => '没有可用字幕';
-	@override String get noAudioTracksAvailable => '没有可用音轨';
-	@override String get noTracksAvailable => '没有可用音轨';
 	@override String get subtitleDownloaded => '字幕已下载';
 	@override String get subtitleDownloadedNotApplied => '字幕已下载，但无法选择';
 	@override String get subtitleDownloadFailed => '字幕下载失败';
@@ -2709,12 +2706,9 @@ extension on TranslationsZh {
 			'videoControls.searchSubtitles' => '搜索字幕',
 			'videoControls.language' => '语言',
 			'videoControls.noSubtitlesFound' => '未找到字幕',
-			'videoControls.noSubtitlesAvailable' => '没有可用字幕',
+			'videoControls.subtitleDownloaded' => '字幕已下载',
 			_ => null,
 		} ?? switch (path) {
-			'videoControls.noAudioTracksAvailable' => '没有可用音轨',
-			'videoControls.noTracksAvailable' => '没有可用音轨',
-			'videoControls.subtitleDownloaded' => '字幕已下载',
 			'videoControls.subtitleDownloadedNotApplied' => '字幕已下载，但无法选择',
 			'videoControls.subtitleDownloadFailed' => '字幕下载失败',
 			'videoControls.searchLanguages' => '搜索语言…',
@@ -3224,11 +3218,11 @@ extension on TranslationsZh {
 			'watchTogether.codeMustBe5Chars' => '会话代码必须是 5 个字符',
 			'watchTogether.joinInstructions' => '输入主持人的会话代码以加入。',
 			'watchTogether.failedToCreate' => '创建会话失败',
-			_ => null,
-		} ?? switch (path) {
 			'watchTogether.failedToJoin' => '加入会话失败',
 			'watchTogether.sessionCodeCopied' => '会话代码已复制到剪贴板',
 			'watchTogether.relayUnreachable' => '无法访问中继服务器。网络运营商的屏蔽可能导致“一起看”不可用。',
+			_ => null,
+		} ?? switch (path) {
 			'watchTogether.reconnectingToHost' => '正在重新连接到主持人…',
 			'watchTogether.currentPlayback' => '当前播放',
 			'watchTogether.joinCurrentPlayback' => '加入当前播放',