feat(gui,fuse,core): album covers as song thumbnails

A song in a listing showed a generic icon, and the player a music note.
Both now show the song's cover: the picture in its tags, read with
lofty, or else a cover, folder, front or album image in its folder.

Only the tags are downloaded. A song up to 4 MiB comes whole. A larger
one brings the ID3v2 tag or the FLAC metadata blocks at its start, an
M4A's moov box, the first 4 MiB of an Ogg file, and the last 4 MiB for
formats that keep their tags at the end. The parts are laid out in a
sparse staging file at their own offsets, as video samples are.

A folder cover comes from that image's cached thumbnail, the thumbnail
its uploader attached, or the image itself. It is stored under the song.
Uploads of songs carry their embedded cover as the Drive thumbnail.

docs/CHANGELOG.md

@@ -104,6 +104,14 @@ Existing folders stay device backups. Older builds cannot open the upgraded data
   10 seconds, and `M` to mute. The volume carries over from one video to the next, and the
   thumbnail strip in Photos stays off a video, where it covered the seek bar.
 
+- **Album covers for songs.** A song in My files, in search and in Shared shows its cover as its
+  thumbnail, and the player shows it above the controls instead of a music note. The cover is the
+  picture in the song's tags (MP3, FLAC, Ogg, Opus, M4A, WAV, AIFF, WavPack, Monkey's Audio), or
+  else a `cover`, `folder`, `front` or `album` image (`.jpg`, `.png`, `.webp`) in the same folder.
+  Only the song's tags are downloaded for it, not the whole song. Songs uploaded from this computer
+  carry their cover as their thumbnail, so they show it at once on other devices too. A cover
+  image added to a folder later is picked up once the song changes or the service restarts.
+
 ### Changed
 
 - **`proton-sdk` and `proton-drive-rs` 0.11.0.** Timestamps are converted where they enter