improved file input and error handling

[git-lakshy]

In this PR

File upload and editor drag&drop handlers used readAsText() for all files, reading binary audio data as text, File read errors were also silent. This PR changes audio file handling into a reusable utility, fixes binary file corruption, and adds proper error feedback.

Changes

• New reusable utility src/audioFileUtils.ts

  • with AudioFile() (now case-insensitive regex check) and readFileContent() (now returns string for code, Uint8Array for audiofiles).

• in FileManager.ts

  • Used src/audioFileUtils.ts to replace the inline regex checks with isAudioFile() and added Error Handling.

• In index.ts

  • Fixed binary corruption for Drag&Drop and upload handler
  • Added error handling for both handlers
  • Export now inlcudes mp3, ogg and aac
  • Early return guard for cancelled upload

• Some minor changes

  • Audio file regex check is now case insensitive
  • Added accept in index.html for better UX, user can still switch file picker to All files
  • Audio files no longer recompile

[git-lakshy]

@sletz can you review this PR

[git-lakshy]

Sorry I thought export failing with an audio file was because of this PR but it was present before this PR

Making the PR ready for review again