Fix palette thumbnails: white-fill curved SVGs + invert filter for dark bg

- Change all curved conveyor/chute SVGs from black to white fill
- Add CSS filter:invert(1) to palette thumbnails so white-fill symbols
  remain visible on the dark palette background

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

svelte-app/src/components/Palette.svelte

@@ -204,6 +204,7 @@
     border-radius: 3px;
     padding: 2px;
     flex-shrink: 0;
+    filter: invert(1);
   }
 
   .palette-item span {