Fix PiP not finding video in iframes/shadow DOM

[Amitind]

Summary

• Picture-in-Picture only searched document.querySelectorAll("video") on the top-level document, so it reported "No video found" on sites that embed the player in an iframe or shadow DOM (common embeds, Reddit/Twitch-style custom players).
• Now runs the PiP script in all frames (allFrames: true) and recursively walks open shadow roots for <video>, picking whichever frame actually found/entered a video.

Test plan

• Load unpacked extension, click PiP on a page with a directly-embedded <video> (regression check)
• Click PiP on a page where the video lives inside an iframe
• Click PiP on a page using shadow-DOM custom video controls