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/README.md

@@ -73,6 +73,7 @@ inspector or `theme_type_variation` in code:
 | `HighlightLarge`, `HighlightMedium` | Label | Orange text: sync headers and every other sync row |
 | `SongRowLabel`, `SongRowLabelPlaying` | Label | A song's text, and the one that is playing |
 | `TabButton` | Button | The Quiz and Statistics tabs; the chosen one is orange |
+| `SoundtrackCover` | Panel | A soundtrack's cover on a statistics card and in its popup: the colour behind the initials until the cover arrives, and the rounded corners the cover is cut to |
 | `StatisticsCard`, `StatisticsCardHover` | PanelContainer | A soundtrack card in the statistics, and the one under the mouse |
 | `StatisticsGrid` | GridContainer | The grid of soundtrack cards, with the space between them |
 | `PlayerBar` | PanelContainer | The music player's rounded bar |