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_song_list.gd

@@ -5,6 +5,10 @@ extends RefCounted
 ## The [code]SoundtrackWithSongList[/code] schema of the MusicServer API.
 
 var added: String
+## 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 last_played: String
 var name: String
 var songs: Array[Song]
@@ -16,6 +20,8 @@ var uuid: String
 static func from_dict(data: Dictionary) -> SoundtrackWithSongList:
 	var result: SoundtrackWithSongList = SoundtrackWithSongList.new()
 	result.added = ApiJson.get_string(data, "added", "")
+	result.alternative_names.assign(ApiJson.get_array(data, "alternative_names"))
+	result.cover_modified = ApiJson.get_int(data, "cover_modified", 0)
 	result.last_played = ApiJson.get_string(data, "last_played", "")
 	result.name = ApiJson.get_string(data, "name", "")
 	for item: Variant in ApiJson.get_array(data, "songs"):
@@ -30,6 +36,8 @@ static func from_dict(data: Dictionary) -> SoundtrackWithSongList:
 func to_dict() -> Dictionary:
 	return {
 		"added": added,
+		"alternative_names": alternative_names,
+		"cover_modified": cover_modified,
 		"last_played": last_played,
 		"name": name,
 		"songs": songs.map(func(item: Song) -> Dictionary: return item.to_dict()),