Fixed theater mode on YouTube (and players with a similar structure).

• The fullscreen button and the F key now open theater mode correctly on
YouTube — previously you'd get only a dimmed page with no video. YouTube
requests fullscreen on the page root, which the extension mistakenly
treated as the player; it now finds the actual player container.
• Theater mode now elevates the real player container, so YouTube's own
controls (seek bar, volume, settings, fullscreen) stay visible and
clickable inside the theater view.
• Videos wrapped in collapsed containers (a technique YouTube and some
custom players use) no longer shrink to zero height in theater mode.
• The theater view now reliably covers site headers and sidebars that
could previously paint on top of the video.