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/stores/serverStore.ts

@@ -19,6 +19,12 @@ interface ServerStore {
 
   crossfadeMs: number;
   setCrossfadeMs: (value: number) => void;
+
+  normalizeAudio: boolean;
+  setNormalizeAudio: (value: boolean) => void;
+
+  customModelsDir: string | null;
+  setCustomModelsDir: (dir: string | null) => void;
 }
 
 export const useServerStore = create<ServerStore>()(
@@ -41,6 +47,12 @@ export const useServerStore = create<ServerStore>()(
 
       crossfadeMs: 50,
       setCrossfadeMs: (value) => set({ crossfadeMs: value }),
+
+      normalizeAudio: true,
+      setNormalizeAudio: (value) => set({ normalizeAudio: value }),
+
+      customModelsDir: null,
+      setCustomModelsDir: (dir) => set({ customModelsDir: dir }),
     }),
     {
       name: 'voicebox-server',