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>

main/main_window.gd

@@ -47,6 +47,12 @@ func _ready() -> void:
 	quiz.server_changed.connect(_on_music_source_changed)
 	quiz.event_logged.connect(log_popup.add_log_row)
 	statistics_view.soundtrack_selected.connect(soundtrack_stats_popup.show_soundtrack)
+	soundtrack_stats_popup.covers = statistics_view.covers
+	# New alternative names are found by the statistics search and the Search popup
+	soundtrack_stats_popup.alternative_names_changed.connect(
+			statistics_view.update_alternative_names)
+	soundtrack_stats_popup.alternative_names_changed.connect(
+			func(_soundtrack: Soundtrack) -> void: search_popup.get_list_of_soundtracks())
 
 	# A saved quiz may have been played from a music folder
 	top_bar.update_offline_state()