feat: Add Audio Description Support for Video Accessibility

[abhalsod-sonata]

Description:
This PR implements audio description (AD) functionality for video content in Open edX, enhancing accessibility compliance with WCAG 2.1 Success Criterion 1.2.5 (Audio Description - Prerecorded). Audio descriptions provide narrated explanations of visual elements in videos, making content accessible to users with visual impairments.

Key Changes:

• Frontend Template (lms/templates/video.html): Added a conditional <audio> element for audio description tracks, rendered only when the feature is enabled.
• Video Initialization (xmodule/js/src/video/01_initialize.js): Extended configuration to handle audio description preferences, including persistence via localStorage and fallback to server-side user state.
• New Audio Description Module (xmodule/js/src/video/09_video_audio_description.js): A comprehensive JavaScript module that:
  • Manages a secondary <audio> element synchronized with the main video.
  • Provides a toggle button in the video controls for enabling/disabling AD.
  • Automatically mutes the original video audio when AD is active to prevent overlap.
  • Handles playback synchronization, seeking, speed changes, and volume adjustments.
  • Persists user preferences and integrates with the existing save state system.
• Module Integration (xmodule/js/src/video/10_main.js): Registered the new VideoAudioDescription module in the video player initialization sequence.
• Styling (xmodule/static/css-builtin-blocks/VideoBlockDisplay.css): Added CSS for the AD toggle button, including active/disabled states, hover/focus effects, and proper positioning in the secondary controls.

How It Works:

• When an audio description source URL is configured for a video block, a toggle button appears in the video controls.
• Activating AD mutes the video's original audio and plays the synchronized description track.
• The AD audio stays in sync with video playback, seeking, pausing, and speed changes.
• User preferences are saved and restored across sessions.
• If no AD source is available, the button is rendered in a disabled state for discoverability.

Testing Notes:

• Verified integration with existing video controls (play/pause, seek, speed, volume).
• Tested synchronization accuracy and drift correction during playback.
• Ensured proper cleanup on player destruction and state persistence.
• Compatible with both HTML5 and YouTube video players.

Related Ticket: TNL2-576

This feature improves the learning experience for visually impaired users by providing essential context for video-based educational content.

[abhalsod-sonata]

Close this #222 PR due to reason is same changes merge on #223 PR.