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/fakes/fake_http_transport.gd

@@ -43,6 +43,7 @@ var queue_id: String = ""
 
 var _responses: Dictionary = {}
 var _successes_left: Dictionary = {}
+var _failures_left: Dictionary = {}
 var _held_paths: Dictionary = {}
 var _previous_transport: HttpTransport
 var _previous_queue_ids: Dictionary[String, String]
@@ -89,6 +90,11 @@ func respond_bytes(path: String, body: PackedByteArray, response_code: int = 200
 	_successes_left.erase(path)
 
 
+## Answers the next [param failures] requests to [param path] with a 503, then as usual.
+func fail_first(path: String, failures: int) -> void:
+	_failures_left[path] = failures
+
+
 ## Lets [param successes] more requests to [param path] succeed, then answers 503 until
 ## a new response is registered for the path.
 func fail_after(path: String, successes: int) -> void:
@@ -156,6 +162,10 @@ func _answer(path: String, on_complete: Callable, follow_redirects: bool = true)
 	var headers: PackedStringArray = []
 	if !queue_id.is_empty() && path != "/token":
 		headers.append("X-Queue-Id: " + queue_id)
+	if _failures_left.get(path, 0) > 0:
+		_failures_left[path] -= 1
+		on_complete.call(503, PackedByteArray(), headers)
+		return
 	if _successes_left.has(path):
 		if _successes_left[path] == 0:
 			on_complete.call(503, PackedByteArray(), headers)