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>

popups/SettingsPopup.tscn

@@ -253,6 +253,27 @@ layout_mode = 2
 focus_mode = 0
 text = "Open folder"
 
+[node name="ClearCacheRow" type="HBoxContainer" parent="SettingsPanel/VBoxContainer"]
+layout_mode = 2
+
+[node name="Title" type="Label" parent="SettingsPanel/VBoxContainer/ClearCacheRow"]
+layout_mode = 2
+size_flags_horizontal = 3
+theme_type_variation = &"SettingsRowLabel"
+text = "Downloaded pictures and songs"
+
+[node name="CacheSizeLabel" type="Label" parent="SettingsPanel/VBoxContainer/ClearCacheRow"]
+unique_name_in_owner = true
+layout_mode = 2
+
+[node name="ClearCacheButton" type="Button" parent="SettingsPanel/VBoxContainer/ClearCacheRow"]
+unique_name_in_owner = true
+layout_mode = 2
+tooltip_text = "Deletes the downloaded character pictures, covers and songs.
+They are downloaded again when needed. Songs in the current quiz are kept."
+focus_mode = 0
+text = "Clear cache"
+
 [node name="FolderDialog" type="FileDialog" parent="."]
 unique_name_in_owner = true
 title = "Choose a music folder"