Fix bootstrap script ffmpeg extraction + stabilize Demucs stem filenames

[ahetrick12]

Summary

Fixes two issues:

• Bootstrap failures during the Windows ffmpeg extraction step
• Split/download failures caused by problematic source filename patterns (e.g., whitespace before extension)

Changes

Updated download-third-party-apps.js:

• Added a guard in moveDirChildrenUpAndDeleteDir to skip gracefully when the source directory does not exist.
• Replaced hardcoded ffmpeg folder name handling with dynamic discovery of extracted directory matching ffmpeg-* and *essentials_build.
• Added explicit error when no extracted ffmpeg directory is found after unzip.

Updated processQueue.js:

• Added explicit Demucs filename template: --filename {stem}.{ext} to normalize stem filenames and avoid inherited problematic naming patterns.
• Hardened Demucs output path resolution in findDemucsOutputDir so it also accepts files written directly in the base output directory (not only nested subdirectory layout).

Testing

Tested only on Windows 11

• Cloned fresh repository and ran npm run download-third-party-apps, confirmed no failures
• Confirmed problematic file names no longer cause failures
  • Tested with test .wav filename; fails on latest release, succeeds with these changes