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>

tests/fakes/ui_tour.gd

@@ -91,6 +91,12 @@ static func make_server() -> FakeHttpTransport:
 	server.respond_bytes("/sync", PackedByteArray())
 	server.respond_json("/sync/progress", SYNC_PROGRESS)
 	server.respond_json("/statistics/soundtracks", soundtrack_statistics())
+	# The character pictures stand in for covers
+	var pictures: Array[String] = character_names()
+	for i: int in SOUNDTRACKS.size():
+		server.respond_bytes("/soundtracks/soundtrack-%d/cover" % i,
+				FileAccess.get_file_as_bytes(CHARACTERS_DIR.path_join(
+						pictures[i % pictures.size()])))
 	server.respond_json("/version/history", SERVER_HISTORY)
 	return server
 
@@ -118,6 +124,11 @@ static func soundtrack_statistics() -> Array[Dictionary]:
 		}
 		if played > 0:
 			soundtrack.soundtrack_last_played = "2026-09-%02dT20:00:00Z" % (i + 1)
+		# Every other soundtrack has a cover, the rest show their initials
+		if i % 2 == 0:
+			soundtrack.cover_modified = 1700000000
+		if i == 0:
+			soundtrack.alternative_names = ["CT", "クロノ・トリガー"]
 		statistics.append(soundtrack)
 	return statistics
 
@@ -292,6 +303,11 @@ func run(on_state: Callable) -> void:
 			break
 	await _settle()
 	await on_state.call("popup_SoundtrackStats", stats_popup)
+	stats_popup.edit_names()
+	stats_popup.new_name_field.text = "Chrono"
+	await _settle()
+	await on_state.call("popup_SoundtrackStats_names", stats_popup)
+	stats_popup.stop_editing()
 	stats_popup.close()
 	_main.show_tab(&"quiz")
 	await _settle()