a11y: label injected controls and preserve native focus

[imparable2022]

Stacked PR

Depends on #4333 (and therefore #4332). This PR is a draft while the preceding accessibility layers are reviewed.

The new change to review here is commit 45e6b2fc5828c1807e1d1eaa1141872c54fe2149.

Problem

ImprovedTube injects icon-only/custom controls into YouTube that do not always expose stable accessible names. Separately, upstream code temporarily replaces HTMLElement.prototype.focus, which can interfere with keyboard and screen-reader focus at a global page level.

What changes

Page-world accessibility

• add js&css/web-accessible/accessibility.js
• derive stable names from ARIA/text/title/tooltip data and known ImprovedTube classes
• hide decorative SVGs
• add visible focus outlines to ImprovedTube controls
• title ImprovedTube settings iframes
• expose tooltip/title success/error feedback through a polite status region
• observe dynamically inserted/updated controls

Wiring

• add the module to manifest.json web-accessible resources
• load it before page-world init.js

Preserve native focus

• remove active ImprovedTube.forbidFocus(2100) usage
• neutralize the legacy global HTMLElement.prototype.focus replacement in appearance.js
• prevent shortcuts.js from assigning HTMLElement.prototype.focus as part of that workaround

The injected-control module is additive. The focus-safety part restores native browser focus semantics rather than introducing another focus model.