Convert MusicPlayer from SDL_mixer to GStreamer backend

[Copilot]

Replaces SDL_mixer audio backend with GStreamer while maintaining full API compatibility with existing MusicPlayerComponent and all playback controls.

Core Changes

GStreamer Pipeline Integration

• Replaced Mix_Music* with GStreamer playbin element
• Added createPipeline() / destroyPipeline() lifecycle management
• Implemented busCallback() for EOS and error handling

Playback Methods

• playMusic(): Uses gst_element_set_state(GST_STATE_PLAYING) + URI setting
• pause/resumeMusic(): State transitions via GST_STATE_PAUSED
• stopMusic(): Sets GST_STATE_NULL
• Seek support via gst_element_seek_simple()

Volume Control

• setGstVolume() wraps playbin volume property
• Maintains 0-128 range for API compatibility (normalized to 0.0-1.0 for GStreamer)
• Threaded fadeToVolume() now modulates GStreamer volume

Position Queries

• getCurrent() / getDuration(): Use gst_element_query_position/duration()
• Returns values in seconds (converted from GST_FORMAT_TIME)

State Queries

• isPlaying() / isPaused(): Query GstState via gst_element_get_state()

Unchanged

• All public method signatures
• Track metadata and album art extraction (ID3 parsing)
• Playlist management (M3U, folder loading)
• Shuffle/loop/fade modes
• Audio visualization infrastructure

Example Usage

// API remains identical - existing code works unchanged
musicPlayer->playMusic(0);           // Play first track
musicPlayer->setVolume(64);          // 50% volume
musicPlayer->nextTrack(1500);        // Fade to next track
double pos = musicPlayer->getCurrent(); // Get position in seconds

Original prompt

convert MusicPlayer class to gstreamer, maintaining compatibility with existing API (especially calls from MusicPlayerComponent, controls like nextTrack prevTrack pause etc.)

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