Fix WAVE accessibility errors for local video generation

[Copilot]

Problem

When generating local videos using the generate() method, the WAVE accessibility checker reported two errors:

1. Missing alternative text - <media-poster> elements with empty alt="" attributes
2. Broken ARIA menu - <media-player> missing proper ARIA labeling for local videos

Example code that triggered the errors:

$localVideo = new Video($mp4_video, 'Eigenes Video');
echo $localVideo->generate();

Solution

This PR implements two minimal fixes to resolve both accessibility issues:

1. Add aria-label to local video/audio players

For local videos and audio files, the <media-player> element now includes an aria-label attribute:

• Uses the video title as the label when available
• Falls back to a generic localized label ("Video-Player" or "Audio-Player") when no title is provided
• Provides proper ARIA context for the <media-video-layout> component

Before:

<media-player title="Eigenes Video" src="..." crossorigin>

After:

<media-player title="Eigenes Video" src="..." aria-label="Eigenes Video" crossorigin>

2. Poster alt text fallback

When a poster is set without alt text via setPoster($src, ''), the video title is now used as a fallback:

• Custom alt text provided via setPoster($src, $alt) is still preserved
• Ensures all poster images have meaningful alternative text

Before:

<media-poster src="poster.jpg" alt="">

After:

<media-poster src="poster.jpg" alt="Eigenes Video">

Impact

• ✅ Zero breaking changes - All existing code continues to work unchanged
• ✅ Automatic improvement - No code changes required by users
• ✅ Better accessibility - Resolves WAVE errors for all local video/audio players
• ✅ Maintains flexibility - Custom alt text still works when explicitly provided

Testing

Verified across multiple scenarios:

• Local video with title → aria-label uses title
• Local video without title → aria-label uses generic label
• Poster with empty alt → Uses title as fallback
• Poster with custom alt → Preserves custom alt text

All WAVE accessibility errors are now resolved.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Wave Fehler</issue_title>
<issue_description>Alles aktuell.

Wenn ich ein Video so ausgebe:

// Lokales Video $localVideo = new Video($mp4_video, 'Eigenes Video'); echo $localVideo->generate();

Erhalte ich mit Wave folgende Fehler:

Kann ich das irgendwie verhindern?</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #46

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[Copilot]

Pull Request Overview

This PR addresses WAVE accessibility errors when generating local video content by implementing proper ARIA labeling and alternative text handling. The changes ensure compliance with accessibility standards while maintaining backward compatibility.

• Add aria-label attributes to local video/audio players for proper screen reader support
• Implement fallback alt text for poster images when empty alt attributes are provided
• Maintain existing functionality while improving accessibility compliance

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