fix: 文の途中から連続再生した際、次以降の文が途中から再生される問題を修正

[akikukeo]

内容

文の途中から連続再生すると、次以降の文も途中から再生される問題を修正します。

本修正により、次以降の文は常に文頭から再生されます。

関連 Issue

close #2634

スクリーンショット・動画など

その他

[voicevox-preview-pages]

🚀 プレビュー用ページを作成しました 🚀

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更新時点でのコミットハッシュ：ad4f165

[sevenc-nanashi]

特に問題なさそう。

動作確認したらマージします。

[Copilot]

Pull request overview

This PR fixes issue #2634 where continuous playback starting from the middle of a sentence would cause subsequent sentences to also start playing from the same offset position instead of from the beginning. The fix resets the audio play start point to undefined (which means play from the beginning) when transitioning to a different audio item during continuous playback.

Key changes:

• Added logic to reset the play start point when the audio key changes during continuous playback
• The original play start point is preserved and restored after continuous playback completes

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[Copilot]

The logic to reset the audio play start point appears in both the playAudioBlob callback and the waitstart event listener. This creates redundancy since playstart (which calls playAudioBlob) always follows waitstart in the event flow. Consider removing the duplicate logic from one of these locations to improve maintainability. The reset in playAudioBlob should be sufficient since it's called whenever audio starts playing.

Suggested change

	if (currentAudioKey !== e.audioKey) {
	mutations.SET_AUDIO_PLAY_START_POINT({ startPoint: undefined });
	}