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>

statistics/StatisticsView.tscn

@@ -107,6 +107,17 @@ popup/item_2/id = 2
 popup/item_3/text = "Least played"
 popup/item_3/id = 3
 
+[node name="AddSeparator" type="VSeparator" parent="Toolbar"]
+layout_mode = 2
+
+[node name="AddSoundtrackButton" type="Button" parent="Toolbar"]
+unique_name_in_owner = true
+layout_mode = 2
+tooltip_text = "Upload a folder of songs to the server as a new soundtrack.
+You can also drop a folder anywhere on the window."
+focus_mode = 0
+text = "Add soundtrack…"
+
 [node name="ScrollContainer" type="ScrollContainer" parent="." unique_id=1212000010]
 layout_mode = 1
 anchors_preset = 15