encapsulating media engine

[Lori-Shu]

No description provided.

[pullfrog]

Important

This PR introduces a major architectural refactor, replacing the internal decode_engine with a new media_engine library.

Reviewed changes

• Architectural Refactor: Removed the decode_engine module and encapsulated media handling into a new external/internal library media_engine.
• UI Enhancements: Added frequency analysis and mel-bar visualization in body_ui.rs and presentation.rs.
• System Integration: Updated appui.rs, controlbar_ui.rs, and whispercpp_transcriber.rs to interface with the new media_engine instead of the old decode_engine.

The refactor appears sound and aligns with the goal of encapsulating media handling. Given the critical nature of the media playback and synchronization flows now handled by media_engine, please ensure thorough testing of:

1. Audio/Video synchronization.
2. Seeking behavior (as seen in controlbar_ui.rs changes).
3. Media file loading and reset_input behavior.

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