Fix #4003: cinema mode / auto cinema mode shows blank screen

[andrianbalanesq]

Closes #4003

Problem

When Auto cinema mode is enabled and a user clicks a YouTube video, the
player area renders as a blank black rectangle and the video does not load.

Root cause

Two defects in createOverlay() and playerCinemaModeEnable() combine
to produce this:

1. The overlay was created with rgba(0, 0, 0, 1) — fully opaque. In
   Firefox (and any browser where YouTube applies transform /
   will-change to #full-bleed-container or an ancestor), the
   position: fixed overlay becomes containing-block-relative and ends
   up sitting above the player, swallowing the video behind a solid black
   rectangle.
2. The overlay had pointer-events: auto (the default), so even when
   the z-index resolution was correct it could intercept clicks meant
   for the play button — explaining the "videos not loading" symptom.

Fix

• Switch the overlay to a translucent dim (rgba(0, 0, 0, 0.7)) so the
  video shows through even if the overlay lands above the player.
• Add pointer-events: none so the overlay never blocks clicks.
• Move the overlay from #full-bleed-container to document.body so
  its position: fixed stays rooted in the viewport regardless of
  YouTube's ancestor transforms.
• Reorder playerCinemaModeEnable() to bring the player to z-index
  10000 BEFORE creating the overlay, so there is never a paint frame
  where the overlay sits on top of an unstyled player.

Tests

tests/unit/cinema-mode-bug.test.js (7 tests) — locks in the four
properties above plus the enable/disable symmetry.

[ImprovedTube2]

the player was meant to have a higher z-Index than the curtain covering the rest of the page

[andrianbalanesq]

Thanks for the direction! You are right — the curtain should stay opaque and cover the page, the player just needs to sit above it.

Simplified the fix:

• Reverted the overlay back to opaque rgba(0, 0, 0, 1) — it is a curtain, not a dim
• Removed pointer-events: none and the document.body append — those were overcomplicating it
• The actual fix: set the player z-index (10000) before creating the overlay (9999), so the player is always on top from the first paint frame

Pushed to the same branch. Let me know if anything else needs adjusting! ✨