feat(mining): add a screenshot frame picker to media review (#254)

changes/media-review-frame-picker-docs.md

@@ -0,0 +1,4 @@
+type: docs
+area: mining
+
+- Document independent screenshot selection in media timing review.

changes/media-review-frame-picker.md

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+type: added
+area: mining
+
+- Choose a still screenshot independently of the audio in media timing review, with a live preview, a separate time slider, and decoded-frame stepping. Local video and seekable remote streams such as Jellyfin are supported.