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>

server/generated/cover_source.gd

@@ -0,0 +1,26 @@
+# Generated by tools/generate_api_types.gd from server/openapi.json.
+# Do not edit; change the server and generate again.
+class_name CoverSource
+extends RefCounted
+## The [code]CoverSource[/code] schema of the MusicServer API.
+
+## The game's ID on SteamGridDB, or the anime's ID on AniList
+var id: String
+## Where the cover comes from
+var source: String
+
+
+## Reads the fields from parsed JSON. Missing fields keep their defaults.
+static func from_dict(data: Dictionary) -> CoverSource:
+	var result: CoverSource = CoverSource.new()
+	result.id = ApiJson.get_string(data, "id", "")
+	result.source = ApiJson.get_string(data, "source", "")
+	return result
+
+
+## Returns the fields under their JSON names, ready for [method JSON.stringify].
+func to_dict() -> Dictionary:
+	return {
+		"id": id,
+		"source": source,
+	}