fix: strip leading dot from extension when checking audio format on upload

[octo-patch]

Fixes #553

Problem

When a user uploads an MP3 or WAV file, the upload handler gets stuck in a
reload loop and the audio is never processed.

Root cause: os.path.splitext() returns the file extension with a leading
dot (e.g. '.mp3'), but allowed_audio_formats in config.yaml stores
values without the dot (e.g. 'mp3'). The comparison

if ext.lower() in load_key("allowed_audio_formats"):   # '.mp3' in ['mp3', 'wav', ...]

always evaluates to False, so convert_audio_to_video() is never called.
The audio file is saved to the output folder but find_video_files() ignores
it (it only matches video extensions), so the UI loops endlessly on the upload
widget.

This also explains why renaming the file to .mp4 "works" — the video-format
branch is hit instead and find_video_files() picks it up.

Solution

Strip the leading dot with [1:] before the membership test, consistent with
how find_video_files() already handles extensions:

if ext.lower()[1:] in load_key("allowed_audio_formats"):

Testing

• Uploaded an .mp3 file: the UI now calls convert_audio_to_video, creates
  black_screen.mp4, removes the original audio file, and re-renders showing
  the video player as expected.
• Uploaded .wav, .flac, .m4a files: same behaviour.
• Uploaded a .mp4 file: unaffected, works as before.