fixes:

- make prefix imported games paths relative to prefix
- show placeholder icon as fallback
- fix crossed working/exe directories
- fix broken working dir in prefix imported games

src/library.js

@@ -93,6 +93,14 @@ function updateGame(id, patch) {
   const games = loadGames()
   const g = games.find(x => x.id === id)
   if (!g) return null
+  // prefix imports are pinned: their paths are stored relative to their
+  // prefix, so NOTHING may move them to another prefix or flip their
+  // import source (the patch fields are simply ignored)
+  if (g.importSource === 'prefix' && patch) {
+    patch = { ...patch }
+    delete patch.prefix
+    delete patch.importSource
+  }
   // playSeconds goes to its own tiny file; the registry just mirrors it for
   // anything reading apps.json directly (loadGames prefers the file anyway).
   if (patch.playSeconds !== undefined) writePlaySeconds(id, patch.playSeconds)