Replace fire-and-forget decode_audio() with a streaming Decoder that uses
libswresample to convert planar float to interleaved stereo, fixing the
sped-up audio bug and eliminating multi-GB memory usage for long files.
Add 10-second rewind/fast-forward, stop (pause in place), position
persistence per file via positions.txt, directory scanning with file
switching, embedded album art display, and a progress bar. Handles both
old and new FFmpeg channel layout APIs via version preprocessor check.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
