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/integration/test_theme.gd

@@ -50,6 +50,7 @@ const STYLES: Dictionary = {
 	"ProgressSlider": "HSlider",
 	"VolumeBar": "HSlider",
 	"StatisticsCard": "PanelContainer",
+	"SoundtrackCover": "Panel",
 	"StatisticsCardHover": "PanelContainer",
 	"StatisticsGrid": "GridContainer",
 	"TabButton": "Button",