Add image format conversion support to local asset flow

The local asset flow (`--local-assets` option) currently does not support automatic conversion of modern image formats (webp, avif, heic) to PNG, while the remote download flow has this capability. This creates an inconsistency where users get different behavior depending on whether assets are sourced locally or downloaded from remote URLs.

## Expected Behavior
Both local and remote asset flows should behave consistently:
- When `--images-to-png=true` (default), both flows should convert webp/avif/heic images to PNG
- Conversion should respect `DO_NOT_CONVERT_EXTENSIONS` (jpg, jpeg, png, gif, svg, ico should not be converted)
- If conversion fails, fall back to copying the original file with a warning