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>

common/file_cache.gd

@@ -0,0 +1,117 @@
+class_name FileCache
+extends RefCounted
+## Downloaded files kept in a folder next to the settings, with an [code]index.json[/code]
+## that records for each file when the server last changed it and when it was
+## downloaded:
+## [codeblock]
+## {"Crono.png": {"modified": 1730000000, "downloaded": 1730100000}}
+## [/codeblock]
+## Used by [CharacterCache] and [CoverCache], which decide when a file is fresh.
+
+const INDEX_FILE_NAME: String = "index.json"
+## The change time of a file when the server can't tell it.
+const UNKNOWN_TIME: int = -1
+
+## The folder's name in [method Settings.config_dir].
+var folder_name: String
+
+# What index.json holds: key -> {"modified": int, "downloaded": int}
+var _index: Dictionary = {}
+
+
+func _init(new_folder_name: String) -> void:
+	folder_name = new_folder_name
+
+
+## The folder the files are kept in.
+func folder() -> String:
+	return Settings.config_dir().path_join(folder_name)
+
+
+## Reads [code]index.json[/code]. A missing or broken index starts empty.
+func load_index() -> void:
+	_index = {}
+	var path: String = folder().path_join(INDEX_FILE_NAME)
+	if FileAccess.file_exists(path):
+		var parsed: Variant = JSON.parse_string(FileAccess.get_file_as_string(path))
+		if parsed is Dictionary:
+			_index = parsed
+
+
+## The keys of the files in the index.
+func keys() -> Array:
+	return _index.keys()
+
+
+## Whether the file for [param key] is in the index and on disk.
+func has_file(key: String) -> bool:
+	return _index.has(key) && FileAccess.file_exists(file_path(key))
+
+
+## When the server last changed the file for [param key], as recorded when it was
+## downloaded, or [constant UNKNOWN_TIME].
+func modified(key: String) -> int:
+	return int(_index.get(key, {}).get("modified", UNKNOWN_TIME))
+
+
+## How many seconds ago the file for [param key] was downloaded.
+func age(key: String) -> float:
+	return Time.get_unix_time_from_system() - float(_index.get(key, {}).get("downloaded", 0))
+
+
+## The file for [param key].
+func read(key: String) -> PackedByteArray:
+	return FileAccess.get_file_as_bytes(file_path(key))
+
+
+## Writes [param bytes] as the file for [param key], last changed on the server at
+## [param modified_time], and saves the index. Returns false if it could not be written.
+func store(key: String, bytes: PackedByteArray, modified_time: int) -> bool:
+	DirAccess.make_dir_recursive_absolute(folder())
+	var file: FileAccess = FileAccess.open(file_path(key), FileAccess.WRITE)
+	if file == null:
+		GameLog.warning("Could not cache ", key, " in ", folder_name, ": ",
+				error_string(FileAccess.get_open_error()))
+		return false
+	file.store_buffer(bytes)
+	file.close()
+	_index[key] = {
+		"modified": modified_time,
+		"downloaded": int(Time.get_unix_time_from_system()),
+	}
+	save_index()
+	return true
+
+
+## Deletes the file for [param key] and its entry, and saves the index.
+func remove(key: String) -> void:
+	DirAccess.remove_absolute(file_path(key))
+	if _index.erase(key):
+		save_index()
+
+
+## Deletes every file whose key is not in [param kept], e.g. files of things the server
+## no longer has.
+func keep_only(kept: Dictionary) -> void:
+	var removed: bool = false
+	for key: String in _index.keys():
+		if !kept.has(key):
+			DirAccess.remove_absolute(file_path(key))
+			_index.erase(key)
+			removed = true
+	if removed:
+		save_index()
+
+
+func save_index() -> void:
+	DirAccess.make_dir_recursive_absolute(folder())
+	var file: FileAccess = FileAccess.open(folder().path_join(INDEX_FILE_NAME),
+			FileAccess.WRITE)
+	if file != null:
+		file.store_string(JSON.stringify(_index, "\t", true))
+
+
+## Where the file for [param key] is. Keys come from the server, so they are made safe
+## as file names.
+func file_path(key: String) -> String:
+	return folder().path_join(key.validate_filename())