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

backend/models.py

@@ -60,6 +60,7 @@ class GenerationRequest(BaseModel):
     engine: Optional[str] = Field(default="qwen", pattern="^(qwen|luxtts|chatterbox|chatterbox_turbo)$")
     max_chunk_chars: int = Field(default=800, ge=100, le=5000, description="Max characters per chunk for long text splitting")
     crossfade_ms: int = Field(default=50, ge=0, le=500, description="Crossfade duration in ms between chunks (0 for hard cut)")
+    normalize: bool = Field(default=True, description="Normalize output audio volume")
 
 
 class GenerationResponse(BaseModel):
@@ -170,6 +171,11 @@ class ModelDownloadRequest(BaseModel):
     model_name: str
 
 
+class ModelMigrateRequest(BaseModel):
+    """Request model for migrating models to a new directory."""
+    destination: str
+
+
 class ActiveDownloadTask(BaseModel):
     """Response model for active download task."""
     model_name: str