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>

server/generated/upload_request.gd

@@ -0,0 +1,22 @@
+# Generated by tools/generate_api_types.gd from server/openapi.json.
+# Do not edit; change the server and generate again.
+class_name UploadRequest
+extends RefCounted
+## The [code]UploadRequest[/code] schema of the MusicServer API.
+
+## Name of the new soundtrack, also its folder name in the music folder
+var name: String
+
+
+## Reads the fields from parsed JSON. Missing fields keep their defaults.
+static func from_dict(data: Dictionary) -> UploadRequest:
+	var result: UploadRequest = UploadRequest.new()
+	result.name = ApiJson.get_string(data, "name", "")
+	return result
+
+
+## Returns the fields under their JSON names, ready for [method JSON.stringify].
+func to_dict() -> Dictionary:
+	return {
+		"name": name,
+	}