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/soundtrack_with_songs.gd

@@ -4,6 +4,10 @@ class_name SoundtrackWithSongs
 extends RefCounted
 ## The [code]SoundtrackWithSongs[/code] schema of the MusicServer API.
 
+## Other names the soundtrack is known by
+var alternative_names: Array[String]
+## When the cover last changed, in Unix seconds. Missing when there is no cover.
+var cover_modified: int
 var songs: Array[SongInfoForStats]
 var soundtrack_id: int
 var soundtrack_last_played: String
@@ -15,6 +19,8 @@ var soundtrack_uuid: String
 ## Reads the fields from parsed JSON. Missing fields keep their defaults.
 static func from_dict(data: Dictionary) -> SoundtrackWithSongs:
 	var result: SoundtrackWithSongs = SoundtrackWithSongs.new()
+	result.alternative_names.assign(ApiJson.get_array(data, "alternative_names"))
+	result.cover_modified = ApiJson.get_int(data, "cover_modified", 0)
 	for item: Variant in ApiJson.get_array(data, "songs"):
 		if item is Dictionary:
 			result.songs.append(SongInfoForStats.from_dict(item))
@@ -29,6 +35,8 @@ static func from_dict(data: Dictionary) -> SoundtrackWithSongs:
 ## Returns the fields under their JSON names, ready for [method JSON.stringify].
 func to_dict() -> Dictionary:
 	return {
+		"alternative_names": alternative_names,
+		"cover_modified": cover_modified,
 		"songs": songs.map(func(item: SongInfoForStats) -> Dictionary: return item.to_dict()),
 		"soundtrack_id": soundtrack_id,
 		"soundtrack_last_played": soundtrack_last_played,