fix(watch-later): inject button outside aria-hidden anchor (#4305)

[shoaibyazdani]

Summary

Fixes #4305.

YouTube marks thumbnail anchors (e.g. <a class="ytLockupViewModelContentImage">) with aria-hidden="true". The Watch Later button was being appended as a direct child of the anchor via thumbnail.appendChild(button), so when a user clicked the button the browser blocked focus with:

Blocked aria-hidden on an element because its descendant retained focus. The focus must not be hidden from assistive technology users.

Appending the button to the thumbnail's parent container (the renderer — ytd-rich-item-renderer / yt-lockup-view-model / etc.) keeps the button focusable while preserving YouTube's a11y semantics.

Why this works

• The button uses position: absolute; top: 4px; right: 4px; and is already positioned relative to a higher ancestor (the renderer). Moving it from the anchor to the renderer's child list doesn't change visual placement.
• findNativeWatchLaterButton(container) works correctly because that helper uses closest() to locate the renderer internally — passing the renderer directly just short-circuits the lookup.
• always mode is unaffected. The existing *:hover>.it-watch-later-button selector still matches because the button's direct parent (the renderer) is now what gets hovered — so hover semantics still work, just with a slightly larger hover area. If reviewers prefer to preserve link-only hover, the selector can be tightened in a follow-up.

Changes

• js&css/extension/www.youtube.com/general/general.js — addWatchLaterButton: append to thumbnail.parentElement instead of thumbnail; track container so the click handler can pass it to findNativeWatchLaterButton (which uses closest() anyway, but passing the renderer explicitly is cleaner).
• tests/unit/watch-later-buttons.test.js — added a test asserting the button is not appended directly to the thumbnail anchor.

Test

PASS tests/unit/watch-later-buttons.test.js
  Watch Later thumbnail buttons
    ✓ registers the feature with init
    ✓ adds a hover and always menu option
    ✓ uses the native Watch Later control before the Innertube fallback
    ✓ styles hover and always visibility states
    ✓ injects button outside the aria-hidden thumbnail anchor (#4305)

Tests:       5 passed, 5 total

Notes

• Minimal, surgical change: one function (addWatchLaterButton) + one click handler reference.
• Independent of PR fix(watch-later): make thumbnail WL button actually add videos #4208 (which addresses the underlying WL button functionality). Both can land separately.