Add a soundtrack by picking or dropping a folder (#50)

"Add soundtrack…" in the statistics toolbar, or a folder dropped anywhere on
the window, opens a popup that lists the folder's songs and uploads them to
the server, which adds the soundtrack at once (Sansan/MusicServer#33).

- The name comes from the folder and can be changed; characters the server
  refuses are replaced.
- Only MP3s are uploaded, as the server takes no other; other song files
  are listed greyed out as left out. Hidden files are skipped.
- Disc folders are flattened with the disc's number first ("1-01 Title.mp3"),
  as the server wants one folder. With one disc folder, names stay as
  they are.
- Songs are sent one at a time with a progress bar, reading one file at a
  time. A failed song is sent again twice, then the upload is cancelled
  and the server's reason shown. Closing the popup cancels it too.
- Not offered while the server syncs, offline, or without MP3s.
- Afterwards search, the inspiration list and the statistics reload.

The API types come from the server's spec (Upload, UploadFile,
UploadRequest).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/integration/test_scene_split.gd

@@ -7,7 +7,7 @@ extends MainWindowTestSuite
 const MAIN_WINDOW_CHILDREN: Array[String] = [
 	"Background", "TopBar", "Quiz", "PopupBackdrop", "SearchPopup", "SyncPopup", "QRPopup",
 	"ShortcutsPopup", "StatisticsView", "AboutPopup", "LogPopup", "SoundtrackStatsPopup",
-	"SettingsPopup", "UpdatePopup",
+	"SettingsPopup", "UpdatePopup", "UploadPopup",
 ]
 
 ## The parts of the quiz and the scene each one comes from.