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>

main/MainWindow.tscn

@@ -13,6 +13,7 @@
 [ext_resource type="PackedScene" path="res://popups/SoundtrackStatsPopup.tscn" id="soundtrack_stats_popup_scene"]
 [ext_resource type="PackedScene" uid="uid://dldpeo5y3l5hq" path="res://popups/SettingsPopup.tscn" id="settings_popup_scene"]
 [ext_resource type="PackedScene" path="res://popups/UpdatePopup.tscn" id="update_popup_scene"]
+[ext_resource type="PackedScene" path="res://popups/UploadPopup.tscn" id="upload_popup_scene"]
 
 [node name="MainWindow" type="Control" unique_id=407149696]
 layout_mode = 3
@@ -119,3 +120,12 @@ offset_left = 640.0
 offset_top = 390.0
 offset_right = 640.0
 offset_bottom = 390.0
+
+[node name="UploadPopup" parent="." instance=ExtResource("upload_popup_scene")]
+unique_name_in_owner = true
+visible = false
+layout_mode = 1
+offset_left = 580.0
+offset_top = 200.0
+offset_right = 580.0
+offset_bottom = 200.0