Show soundtrack covers and edit alternative names (#49)

Covers: statistics cards show the soundtrack's cover on the left, and the
statistics popup shows it large. Until it arrives, and without a cover, the
soundtrack's initials stand in. Covers load only for cards on or near the
screen, three at a time, and are cached in covers/ next to the settings
(CoverCache), downloaded again only when cover_modified changes. The index and
file handling are shared with CharacterCache in a new FileCache.

Alternative names: the popup shows them and has an editor (add, remove, save)
that sends PUT /soundtracks/{uuid}/alternative-names. The statistics search and
the Search popup find soundtracks by them; Search shows the matching name after
the real one. Search now loads /soundtracks, falling back to the plain list.

The API types are regenerated from the server's spec, with a SoundtrackCover
theme style in every theme.

Closes #49

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/fakes/fake_http_transport.gd

@@ -25,6 +25,9 @@ var requested_paths: Array[String] = []
 var requested_methods: Array[int] = []
 ## Headers of each request, in the same order as [member requested_paths].
 var requested_headers: Array[PackedStringArray] = []
+## The JSON body of each request, empty for none, in the same order as
+## [member requested_paths].
+var requested_bodies: Array[String] = []
 ## How many tokens have been asked for.
 var token_requests: int = 0
 ## The JSON body of the last token request.
@@ -120,6 +123,7 @@ func request(
 	requested_paths.append(path)
 	requested_methods.append(method)
 	requested_headers.append(headers)
+	requested_bodies.append(body)
 	if !OPEN_PATHS.has(path.get_slice("?", 0)) && !_has_valid_token(headers):
 		on_complete.call(401, JSON.stringify({"error": "Unauthorized"}).to_utf8_buffer(),
 				PackedStringArray())