[Bug Report] opus files aren't tagged

 Root cause, from the code — two separate gaps stacking on each other:

 1. The ffmpeg command itself never asks to keep metadata. ConvertAudio's opus case only sets -codec:a libopus -b:a <rate> -map 0:a — no -map_metadata 0, and -map 0:a maps only the audio
    stream, excluding FLAC's embedded cover art (which lives as a separate attached-picture stream). Result: raw conversion produces a tag-less, cover-less file.

 2. The app's own metadata re-embedding fallback doesn't recognize .opus as a file extension. After conversion, SpotiFlac is supposed to re-embed title/artist/genre/ISRC/cover/lyrics from
    the original FLAC via EmbedMetadataToConvertedFile() — but that function's format switch only lists .flac, .mp3, .m4a, .ogg. .opus isn't in the list, so it falls through to "unsupported
    file format" and silently fails (just logs a warning, doesn't stop the download). Lyrics embedding has the same gap — EmbedLyricsOnlyUniversal() only supports .mp3, .flac, .m4a; Opus
    isn't handled there either.

 So both the primary path and the intended fallback miss .opus specifically, for what looks like the same underlying reason: the extension-matching code was written against .ogg, but the
 actual files SpotiFlac writes out use the .opus extension.```

### Type

Track

### Spotify URL

any

### Additional Context

`ffmpeg -i input.flac -codec:a libopus -b:a 128k -map 0:a output.opus` produces zero tags. this is what SpotiFLAC runs under the hood, i believe

### SpotiFLAC Version

v7.2.2

### OS

Linux

### Location

Earth