redesign: Can't start playback from Songs tab in offline mode

[rom4nik]

Steps to reproduce (as of 89fec62):

1. Download an album
2. Enable offline mode
3. Go to songs tab
4. Click on a song
5. Playback doesn't start

Logs:

I/flutter (26426): [Chopper/INFO] 2023-12-09 20:07:37.142363: 
I/flutter (26426): [Chopper/INFO] 2023-12-09 20:07:37.142759: --> GET https://example.com/Items/[song1itemid]/InstantMix?userId=[userid]&limit=200
I/flutter (26426): [Chopper/INFO] 2023-12-09 20:07:37.142903: Content-Type: application/json
I/flutter (26426): [Chopper/INFO] 2023-12-09 20:07:37.143022: X-Emby-Authorization: MediaBrowser UserId="[userid]", Client="Finamp", Device="Pixel 3", DeviceId="[deviceid]", Version="0.6.15"
I/flutter (26426): [Chopper/INFO] 2023-12-09 20:07:37.143146: X-Emby-Token: [token]
I/flutter (26426): [Chopper/INFO] 2023-12-09 20:07:37.143278: content-length: 0
I/flutter (26426): [Chopper/INFO] 2023-12-09 20:07:37.143424: --> END GET
I/flutter (26426): [Chopper/INFO] 2023-12-09 20:07:37.495575: 
I/flutter (26426): [Chopper/INFO] 2023-12-09 20:07:37.495956: <-- 200 OK GET https://example.com/Items/[song1itemid]/InstantMix?userId=[userid]&limit=200
I/flutter (26426): [Chopper/INFO] 2023-12-09 20:07:37.496125: x-response-time-ms: 87
I/flutter (26426): [Chopper/INFO] 2023-12-09 20:07:37.496247: content-type: application/json; charset=utf-8
I/flutter (26426): [Chopper/INFO] 2023-12-09 20:07:37.496355: transfer-encoding: chunked
I/flutter (26426): [Chopper/INFO] 2023-12-09 20:07:37.496470: date: Sat, 09 Dec 2023 19:07:37 GMT
I/flutter (26426): [Chopper/INFO] 2023-12-09 20:07:37.496573: strict-transport-security: max-age=16000000; includeSubDomains; always
I/flutter (26426): [Chopper/INFO] 2023-12-09 20:07:37.496703: server: Kestrel
I/flutter (26426): [Chopper/INFO] 2023-12-09 20:07:37.496824: content-length: 297177
I/flutter (26426): [Chopper/INFO] 2023-12-09 20:07:37.496942: 
I/flutter (26426): [Chopper/INFO] 2023-12-09 20:07:37.497054: {"Items":[{"Name":"Dandelion (Pandapush Remix)","ServerId":"[serverid]","Id":"[song1itemid]","PremiereDate":"2021-03-19T00:00:00.0000000Z","ChannelId":null,"RunTimeTicks":1433719680,"ProductionYear":2021,"IndexNumber":2,"ParentIndexNumber":1,"IsFolder":false,"Type":"Audio","ParentLogoItemId":"[artist1itemid]","ParentBackdropItemId":"[artist1itemid]","ParentBackdropImageTags":["[someotherid]"],"UserData":{"PlaybackPositionTicks":0,"PlayCount":17,"IsFavorite":false,"LastPlayedDate":"2023-12-01T13:17:24.0042489Z","Played":true,"Key":"Galantis-Dandelion (Remixes)-0001-0002Dandelion (Pandapush Remix)"},"Artists":["Galantis","Jvke"],"ArtistItems":[{"Name":"Galantis","Id":"[artist1itemid]"},{"Name":"Jvke","Id":"[artist2itemid]"}],"Album":"Dandelion (Remixes)","AlbumId":"[albumitemid]","AlbumPrimaryImageTag":"[sometruncatedid]
I/flutter (26426): [Chopper/INFO] 2023-12-09 20:07:37.505857: <-- END HTTP
I/flutter (26426): [DownloadsHelper/INFO] 2023-12-09 20:07:37.519543: Song [song1itemid] exists offline, using local file, title: Dandelion (Pandapush Remix)
I/flutter (26426): [DownloadsHelper/INFO] 2023-12-09 20:07:37.546731: Song [song2itemid] exists offline, using local file, title: Dandelion (Carneyval Remix)
I/flutter (26426): [MusicPlayerBackgroundTask/INFO] 2023-12-09 20:07:37.561139: Stopping audio service
I/flutter (26426): [QueueService/SEVERE] 2023-12-09 20:07:37.564575: Offline mode enabled but downloaded song not found. title: Shimmer (Feat. Christian Burns)
I/flutter (26426): [AudioServiceHelper/SEVERE] 2023-12-09 20:07:37.564901: Offline mode enabled but downloaded song not found. title: Shimmer (Feat. Christian Burns)
E/flutter (26426): [ERROR:flutter/runtime/dart_vm_initializer.cc(41)] Unhandled Exception: Offline mode enabled but downloaded song not found. title: Shimmer (Feat. Christian Burns)
E/flutter (26426): 

I've redacted the logs a bit, and edited code to include song titles in the errors.

• there are 3 songs downloaded, all from the same 1 album (Dandelion (Remixes))
• song1 is Dandelion (Pandapush Remix), which I clicked on
• song2 is another song from the album - Dandelion (Carneyval Remix)
• Shimmer (Feat. Christian Burns) is a song from another album, which hasn't been downloaded, and its ID likely was received as part of the Instant Mix requested from server.

This issue is probably a regression from #484, see https://github.com/jmshrv/finamp/pull/484/files#diff-8ce322217fed731e5c785038127677c3268344f05cc74b0af807c8534c4b0b37R209 - instead of playing just one song, there's a request for Instant Mix (nice feature btw). From observing how this works in online mode, I guess that for a split second another song is put on top of queue, and then the order is corrected, and the originally clicked song plays?

[Chaphasilor]

Good catch. Yeah it's trying to play an instant mix, I didn't really test much with offline mode. The instant mix is a server-side feature that already has the source song as the first item, so there's no reordering going on, we're taking the order straight from the server.
I got a proof of concept for simply playing a single song (like before the regression), but because it's offline mode we could actually load up all songs and play the tracks in-order. What do you think?

[rom4nik]

It might be a little weird at first glance that playing in order is available only in offline mode, but I have no preference here, at least it would partially help with #502.

[Chaphasilor]

Yeah in the context of #502 I think we should just get full song list playback whereever possible...

[Chaphasilor]

Okay, in offline mode all tracks will now be loaded into the queue in-order, with the selected track being sandwiched somewhere in-between and selected as the current track.
Hopefully I can emulate this in online mode soon.