fix(vc): avoid duplicate output extensions

[leno23]

Summary

• Strip an existing source extension before appending the selected export format in vc_multi
• Avoid writing files like song.wav.wav when the input file is already a WAV/FLAC/etc.
• Preserve the original input path for status reporting by using a separate save_path variable for converted formats

Fixes #2758.

Test Plan

• python3 -m py_compile /tmp/rvc_2758_patchwork/infer/modules/vc/modules.py
• Local filename behavior check:
  • song.wav + wav -> song.wav
  • song.flac + flac -> song.flac
  • song.mp3 + wav -> song.wav
  • dir/song.wav + mp3 -> song.mp3

Notes

• This is a minimal WebUI/export path fix and does not touch model inference or algorithm logic.