Live Photos settle on the captured moment

A Live Photo should honor the instant I meant to capture, not just play a video and stop wherever it happens to end. The slideshow now reaches that key image naturally, rests there, and then finishes the surrounding motion with a gentler transition.

Changelog

• Live Photos now pause naturally on their captured key image between the motion recorded before and after it.
• Smoother transitions and background preparation remove the abrupt snap and black blink around the still-image hold.
• The complete Live Photo motion is preserved even when the captured key image is not exactly halfway through the clip.
• Live Photo motion now blends more gently into and out of the captured still, making differences in video and photo framing feel less abrupt.
• Live Photo motion now blends into and out of the captured still over 350 milliseconds for a gentler framing change.
• Closing Settings now immediately shows a clear saving screen while the slideshow is prepared.