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>

themes/christmas_theme.tres

@@ -1240,6 +1240,14 @@ content_margin_right = 10.0
 content_margin_top = 4.0
 content_margin_bottom = 4.0
 
+[sub_resource type="StyleBoxFlat" id="StyleBoxFlat_soundtrackcover"]
+bg_color = Color(0.361, 0.059, 0.094, 1)
+corner_radius_top_left = 8
+corner_radius_top_right = 8
+corner_radius_bottom_right = 8
+corner_radius_bottom_left = 8
+corner_detail = 8
+
 [resource]
 default_font = ExtResource("1_default_font")
 Button/colors/font_color = Color(0.478, 0.082, 0.125, 1.0)
@@ -1495,6 +1503,8 @@ StepButton/styles/disabled = SubResource("StyleBoxFlat_step_disabled")
 StepButton/styles/hover = SubResource("StyleBoxFlat_step_hover")
 StepButton/styles/normal = SubResource("StyleBoxFlat_step_normal")
 StepButton/styles/pressed = SubResource("StyleBoxFlat_step_pressed")
+SoundtrackCover/base_type = &"Panel"
+SoundtrackCover/styles/panel = SubResource("StyleBoxFlat_soundtrackcover")
 StatisticsCard/base_type = &"PanelContainer"
 StatisticsCard/styles/panel = SubResource("StyleBoxFlat_statscard")
 StatisticsCardHover/base_type = &"PanelContainer"