Fix: extraction 'No matching files found' error + diagnostic logging

- Fix embedding output filename: use os.path.splitext instead of
  dangerous str.replace('wav','npy') which could mangle paths containing
  'wav' in directory names (e.g. /path/to/wav_files/...)
- Add fallback name matching in generate_filelist: if direct
  intersection of wav/feature basenames is empty, try stripping
  .wav suffix from feature names (covers embedders that save with
  different naming conventions)
- Add diagnostic logging when file matching fails: logs contents
  of each directory to make the actual mismatch easy to spot
- Fix extract.py logging 'success' message even when extraction
  actually failed (now only logs success on actual success)

Author: BF667

arvc/app/tabs/downloads/__init__.py

@@ -1,7 +0,0 @@
-"""
-Downloads tab for Advanced RVC Inference Gradio interface.
-"""
-
-from .downloads import download_tab
-
-__all__ = ["download_tab"]