fix: correctly display duration statistic in preview panel

[ludvig-sandh]

Summary

Fixes #1037, where previewing a video/audio didn't display the duration until selected again. Once selected again, duration was displayed, but a million times larger than expected.

The "million times larger" problem came from multiplying the duration in milliseconds by 1000. The fix was to divide by 1000 instead, to get seconds.

The problem of the duration not showing up was due to QMediaPlayer loading the duration asynchronously but the UI's trying to access it immediately. I fixed it by adding a signal that is triggered when the duration is available by QMediaPlayer. This signal causes the UI to update again via a callback, this time including the duration.

Some extra details:

• I return early in the callback if the selected preview file has been changed by the user. This avoids displaying stale preview information.
• The function updating the preview stats (in the UI) is called both immediately and upon the callback, making sure the immediately available data still shows up on screen even if QMediaPlayer would be slow/fail to load the duration.
• Renamed PreviewThumbView.__filepath to PreviewThumbView.__thumb_filepath since we now store both the path to the thumb and media file in that class, and we want to distinguish them. Just makes everything clearer.
• Previously this solution was used for formatting the duration:
  dur_str = str(timedelta(seconds=float(stats.duration)))[:-7]
  I think the idea was to strip out the microseconds (6 digits plus the decimal point). However, converting a timedelta to a string doesn't include microseconds. So this solution incorrectly pruned 7 characters from the valid duration string. I ended up replacing it with my own formatter for clarity.

Tasks Completed

Evidence of correct duration stats:

Also tried quickly selecting different files and verified that always the expected duration showed up on screen.

• Platforms Tested:
  • Windows x86
  • Windows ARM
  • macOS x86
  • macOS ARM
  • Linux x86
  • Linux ARM
• Tested For:
  • Basic functionality
  • PyInstaller executable