Route VTT subtitle output to batch folders and temp fallback for UI remounts

[Copilot]

.vtt subtitles were being written to a global gradio_outputs/subtitles path, separating them from their generated audio batch artifacts and creating avoidable workspace clutter during UI subtitle refreshes. This update aligns subtitle output with per-batch audio output and moves UI-only fallback files to OS temp storage.

• Subtitle output routing

  • lrc_to_vtt_file(...) now accepts output_dir.
  • If provided, VTT is written there; otherwise it falls back to tempfile.gettempdir()/acestep_subtitles (instead of a hardcoded project folder).

• Auto-LRC path alignment

  • _run_auto_lrc(...) now accepts output_dir and forwards it to lrc_to_vtt_file(...).
  • generate_with_progress(...) passes the active temp_dir (batch_[timestamp]) into _run_auto_lrc(...), so auto-generated subtitles are colocated with batch audio.

• Manual LRC path alignment

  • generate_lrc_handler(...) now derives the sample’s batch directory from audio_paths and passes that directory to lrc_to_vtt_file(...).
  • Manual subtitle generation now writes beside the corresponding sample audio instead of a global subtitle directory.

• Focused coverage updates

  • Added assertions for custom output_dir and OS-temp fallback behavior.
  • Added checks that manual and auto LRC paths forward the expected output directory.

def lrc_to_vtt_file(lrc_text: str, total_duration: float = None, output_dir: Optional[str] = None):
    vtt_output_dir = output_dir or os.path.join(tempfile.gettempdir(), "acestep_subtitles")
    ...