fix: make video export work on Android 11+

[cizra]

The original com.arthenica FFmpegKit artifact no longer resolves from Maven Central, so a clean checkout couldn't even build. Once it did build, the export pipeline silently produced empty files, aborted with "No valid media batches available", or hung on stale SAF handles, depending on storage mode and Android version.

Build:

• Switch app/build.gradle to a maintained fork that keeps the com.arthenica.ffmpegkit API intact, and adapt the few callsites that touched FFmpegKit internals.

Export:

• Drop empty/invalid batches before invoking FFmpeg, and fail fast if no usable inputs remain instead of feeding it an empty concat list.
• Reopen SAF input descriptors for each retry; the previous code reused one PFD across all 17 parameter attempts, so once FFmpeg closed it on the first failure the remaining retries had nothing to read.
• Stage all FFmpeg I/O through ContentResolver-opened file descriptors (fd:<n> paths), with Closeable lifetimes tracked by FFmpegInputPaths/FFmpegOutputTarget.
• Fix the MediaStore path-rewrite bug on both sides of the pipeline: MediaStore rewrites relative paths starting with '.' (our '.video_recordings' lives on disk as '_.video_recordings') and prefixes pending files with '.pending--', so the raw File paths we reconstructed never matched the actual files. Inputs and outputs now go through the MediaStore URI, which is why users were seeing abandoned 'tmp--*' content:// entries with no usable output file.

Code changes by Claude Opus running inside Pi.dev. I built and tested this commit, it works fine in my phone.

[cizra]

@Myzel394 may I ask for a review?

[Myzel394]

Hey @cizra,

thanks so much for opening all your PRs! I will take a look at this, but probably not in the near future (maybe around September - October). I'm too busy with other stuff currently. I've also tried updating the app in the past, but it was always so much since the ffmpeg library is deprecated and some stuff changed, and the app itself is also quite complex.

Did you create your PRs with the help of AI? I'm not against the usage of AI, I just want transparency here.

[cizra]

Yes, AI was involved. It's also in the commit message:

Code changes by Claude Opus running inside Pi.dev. I built and tested this commit, it works fine in my phone.

I don't speak Kotlin myself (though I'm a developer).
I pushed a merge of all my work into https://github.com/cizra/Alibi - I've lightly tested this merge of all those PRs.
@Myzel394