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

tauri/src-tauri/tauri.conf.json

@@ -56,7 +56,7 @@
   },
   "plugins": {
     "shell": {
-      "open": true
+      "open": ".*"
     },
     "updater": {
       "pubkey": "dW50cnVzdGVkIGNvbW1lbnQ6IG1pbmlzaWduIHB1YmxpYyBrZXk6IEUxRENBQkRBQjdBNTM1OTIKUldTU05hVzMycXZjNGJGcUxmcVVocll2QjdSaTJNdlFxR2M3VDJsMnVvbDdyZGRPMmRlOW9aWTcK",