Add Clear cache to the settings

A new row, "Downloaded pictures and songs", shows how much space the cached
character pictures, covers and downloaded songs take, with a Clear cache
button. Clearing deletes them, but keeps the songs the current quiz uses,
the settings, config.ini and the saved quiz. The characters are loaded
again at once; covers come back when they are shown.

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

common/character_cache.gd

@@ -34,6 +34,11 @@ static func folder() -> String:
 	return Settings.config_dir().path_join(FOLDER_NAME)
 
 
+## Deletes every cached picture. They are downloaded again when next asked for.
+func clear() -> void:
+	_files.clear()
+
+
 ## Fetches the names of the characters and calls [param on_done] with them as an
 ## [code]Array[String][/code]. Cached pictures of characters no longer on the server are
 ## deleted.

common/cover_cache.gd

@@ -38,6 +38,11 @@ static func folder() -> String:
 	return Settings.config_dir().path_join(FOLDER_NAME)
 
 
+## Deletes every cached cover. They are downloaded again when next asked for.
+func clear() -> void:
+	_files.clear()
+
+
 ## Deletes the covers of soundtracks that are not in [param covers] (UUID -> cover
 ## time), and those whose soundtrack no longer has a cover.
 func keep_only(covers: Dictionary) -> void:

common/file_cache.gd

@@ -103,6 +103,29 @@ func keep_only(kept: Dictionary) -> void:
 		save_index()
 
 
+## Deletes every file in the folder, the index too, e.g. to free the space they take.
+func clear() -> void:
+	_index = {}
+	if !DirAccess.dir_exists_absolute(folder()):
+		return
+	for file_name: String in DirAccess.get_files_at(folder()):
+		DirAccess.remove_absolute(folder().path_join(file_name))
+
+
+## How many bytes the files in [param dir] take, leaving out the paths in [param skip]
+## and any subfolders. 0 when the folder doesn't exist.
+static func folder_size(dir: String, skip: PackedStringArray = []) -> int:
+	if !DirAccess.dir_exists_absolute(dir):
+		return 0
+	var total: int = 0
+	for file_name: String in DirAccess.get_files_at(dir):
+		var path: String = dir.path_join(file_name)
+		var file: FileAccess = FileAccess.open(path, FileAccess.READ)
+		if file != null && !skip.has(path):
+			total += file.get_length()
+	return total
+
+
 func save_index() -> void:
 	DirAccess.make_dir_recursive_absolute(folder())
 	var file: FileAccess = FileAccess.open(folder().path_join(INDEX_FILE_NAME),