fix(utils): correct bbox_inches default from invalid string to None

src/utils/export.py

@@ -20,7 +20,7 @@ def save_chart(
     filename: str,
     output_dir: str = "output/charts",
     dpi: int = EXPORT_DPI,
-    bbox_inches: str = "default",
+    bbox_inches: Optional[str] = None,
 ) -> str:
     """Save a matplotlib figure as high-resolution PNG.
 
@@ -29,7 +29,7 @@ def save_chart(
         filename: Output filename (e.g., '01_shiller_cape.png')
         output_dir: Base output directory
         dpi: Resolution (default 300)
-        bbox_inches: Bbox mode for tight layout
+        bbox_inches: Bbox mode for tight layout (default None, matplotlib auto)
 
     Returns:
         Full path to saved file
@@ -74,7 +74,6 @@ def list_output_charts(output_dir: str = "output/charts") -> list[str]:
 
 def get_chart_metadata(filepath: str) -> dict:
     """Get basic metadata about a saved chart file."""
-    from PIL import Image  # Try PIL first, fallback below
 
     p = Path(filepath)
     if not p.exists():