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>

popups/UploadPopup.tscn

@@ -0,0 +1,122 @@
+[gd_scene format=3]
+
+[ext_resource type="Script" path="res://popups/upload_popup.gd" id="popup_script"]
+
+[node name="UploadPopup" type="Control" node_paths=PackedStringArray("panel")]
+layout_mode = 3
+anchors_preset = 15
+anchor_right = 1.0
+anchor_bottom = 1.0
+grow_horizontal = 2
+grow_vertical = 2
+script = ExtResource("popup_script")
+panel = NodePath("UploadPanel")
+
+[node name="UploadPanel" type="Panel" parent="."]
+unique_name_in_owner = true
+layout_mode = 0
+offset_right = 760.0
+offset_bottom = 680.0
+theme_type_variation = &"PopupBackground"
+
+[node name="Rows" type="VBoxContainer" parent="UploadPanel"]
+layout_mode = 0
+offset_left = 24.0
+offset_top = 16.0
+offset_right = 736.0
+offset_bottom = 664.0
+theme_override_constants/separation = 10
+
+[node name="TitleLabel" type="Label" parent="UploadPanel/Rows"]
+layout_mode = 2
+theme_type_variation = &"Title"
+text = "Add a soundtrack"
+horizontal_alignment = 1
+
+[node name="HelpLabel" type="Label" parent="UploadPanel/Rows"]
+layout_mode = 2
+autowrap_mode = 3
+text = "The songs are uploaded to the server, which adds them as a new soundtrack right away. Songs in disc folders are added too, with the disc's number first."
+
+[node name="NameRow" type="HBoxContainer" parent="UploadPanel/Rows"]
+layout_mode = 2
+theme_override_constants/separation = 10
+
+[node name="NameTitle" type="Label" parent="UploadPanel/Rows/NameRow"]
+layout_mode = 2
+theme_type_variation = &"SettingsRowLabel"
+text = "Name"
+
+[node name="NameField" type="LineEdit" parent="UploadPanel/Rows/NameRow"]
+unique_name_in_owner = true
+layout_mode = 2
+size_flags_horizontal = 3
+placeholder_text = "Soundtrack name"
+max_length = 200
+
+[node name="ChooseFolderButton" type="Button" parent="UploadPanel/Rows/NameRow"]
+unique_name_in_owner = true
+layout_mode = 2
+focus_mode = 0
+text = "Other folder…"
+
+[node name="FolderLabel" type="Label" parent="UploadPanel/Rows"]
+unique_name_in_owner = true
+layout_mode = 2
+theme_type_variation = &"TextMedium"
+text_overrun_behavior = 3
+
+[node name="SongScroll" type="ScrollContainer" parent="UploadPanel/Rows"]
+layout_mode = 2
+size_flags_vertical = 3
+horizontal_scroll_mode = 0
+
+[node name="SongList" type="VBoxContainer" parent="UploadPanel/Rows/SongScroll"]
+unique_name_in_owner = true
+layout_mode = 2
+size_flags_horizontal = 3
+
+[node name="SummaryLabel" type="Label" parent="UploadPanel/Rows"]
+unique_name_in_owner = true
+layout_mode = 2
+autowrap_mode = 3
+
+[node name="UploadProgress" type="ProgressBar" parent="UploadPanel/Rows"]
+unique_name_in_owner = true
+visible = false
+layout_mode = 2
+show_percentage = false
+
+[node name="StatusLabel" type="Label" parent="UploadPanel/Rows"]
+unique_name_in_owner = true
+visible = false
+layout_mode = 2
+theme_type_variation = &"HighlightMedium"
+autowrap_mode = 3
+
+[node name="Buttons" type="HBoxContainer" parent="UploadPanel/Rows"]
+layout_mode = 2
+theme_override_constants/separation = 12
+alignment = 2
+
+[node name="CancelButton" type="Button" parent="UploadPanel/Rows/Buttons"]
+unique_name_in_owner = true
+layout_mode = 2
+focus_mode = 0
+text = "Cancel"
+
+[node name="UploadButton" type="Button" parent="UploadPanel/Rows/Buttons"]
+unique_name_in_owner = true
+layout_mode = 2
+focus_mode = 0
+text = "Upload"
+
+[node name="FolderDialog" type="FileDialog" parent="."]
+unique_name_in_owner = true
+title = "Choose a soundtrack folder"
+initial_position = 2
+size = Vector2i(800, 500)
+ok_button_text = "Select Current Folder"
+file_mode = 2
+access = 2
+use_native_dialog = true