Feature request: Allow selection of external subtitle as a reference

[gwiz84]

First of all, thank you for AutoSubSync. I've been using it together with Plex/Bazarr and it works very well.

While using the application, I ran into an edge case that made me think of a possible improvement.

Currently, if a video contains an embedded subtitle track, AutoSubSync can use that as a reference (as I understand it?). However, many times I already have a correctly timed subtitle file in another language stored separately next to the video.

Example:

Movie.mkv
Movie.en.srt (correct timing)
Movie.da.srt (out of sync)

In this situation, it would be useful to manually select an external subtitle file as the reference source and synchronize the target subtitle against it.

Assuming ffsubsync already supports subtitle-to-subtitle synchronization, perhaps the GUI could expose this workflow more directly.

I think this could help users in cases where:

• The video already has a correctly timed subtitle in another language.
• Audio-based synchronization is less reliable.
• The reference subtitle exists externally rather than embedded in the video.

Just an idea from a happy user.

Thank you so much for this program I absolutely love it!