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/statistics_view.gd

@@ -11,6 +11,8 @@ extends Panel
 
 ## Emitted when a card is clicked, to show all songs of [param soundtrack].
 signal soundtrack_selected(soundtrack: SoundtrackWithSongs)
+## Emitted when Add soundtrack… is pressed.
+signal add_soundtrack_pressed
 
 ## The orders the cards can be sorted in, in the order of the sort menu.
 enum SortOrder { A_TO_Z, Z_TO_A, MOST_PLAYED, LEAST_PLAYED }
@@ -57,6 +59,7 @@ var _songs_ready: bool = false
 @onready var soundtracks_mode_button: Button = %SoundtracksModeButton
 @onready var songs_mode_button: Button = %SongsModeButton
 @onready var search_delay: Timer = %SearchDelay
+@onready var add_soundtrack_button: Button = %AddSoundtrackButton
 
 
 func _ready() -> void:
@@ -69,6 +72,7 @@ func _ready() -> void:
 	show_option.item_selected.connect(func(_index: int) -> void: apply_filter())
 	sort_option.item_selected.connect(func(_index: int) -> void: apply_sort())
 	retry_button.pressed.connect(refresh)
+	add_soundtrack_button.pressed.connect(add_soundtrack_pressed.emit)
 	retry_button.visible = false
 	# The cards have moved by the end of the frame
 	card_scroll.get_v_scroll_bar().value_changed.connect(