Update auto-fill-labels loop video to extend end frame

[jni]

I noticed this when making the social media posts for 0.8.0: the label
fill happens only in the last couple of frames in the video, which on
many players/browsers means that you barely see the label closing flash
on the screen before the video restarts, which is quite jarring. This
updated video adds a pause by replicating the last frame for a couple of
seconds.

[jni]

Compare old video here:

https://napari.org/dev/release/release_0_8_0.html#auto-fill-label-contours-when-painting

with the updated video from this PR here:

https://output.circle-artifacts.com/output/job/078159d2-3e16-4094-a0e5-d281824b70a1/artifacts/0/docs/docs/_build/html/release/release_0_8_0.html#auto-fill-label-contours-when-painting

[TimMonko]

Looks great! How did you do this?

[jni]

I asked Claude how to replicate the last frame for 3s in ffmpeg and it spit out this syntax, which worked:

ffmpeg -i auto-fill-labels.webm -vf "tpad=stop_mode=clone:stop_duration=3" auto-fill-labels-padded.webm