feat: model management improvements and folder migration

- Add model folder migration with byte-level progress tracking (backend + UI)
- Custom models directory support via VOICEBOX_MODELS_DIR env var passed to sidecar
- Hardcoded model descriptions displayed in model detail cards
- Open model folder button in storage location row
- Remove 'not downloaded' badge from model cards
- Fix server settings scroll offset for audio player
- Fix shell open permission to allow file paths
- Add normalize toggle to generation settings

app/src/App.tsx

@@ -94,10 +94,11 @@ function App() {
 
     serverStartingRef.current = true;
     const isRemote = useServerStore.getState().mode === 'remote';
+    const customModelsDir = useServerStore.getState().customModelsDir;
     console.log(`Production mode: Starting bundled server... (remote: ${isRemote})`);
 
     platform.lifecycle
-      .startServer(isRemote)
+      .startServer(isRemote, customModelsDir)
       .then((serverUrl) => {
         console.log('Server is ready at:', serverUrl);
         // Update the server URL in the store with the dynamically assigned port