The settings bundle stores values in the database. When upgrading from
a version without datasheetAsPdf, the stored JSON lacks this key,
causing a TypeError when assigning null to a non-nullable bool.
Making it nullable with a fallback in KiCadHelper fixes the upgrade path.
New setting "Datasheet field links to PDF" in KiCad EDA settings.
When enabled (default), the datasheet field resolves to the actual
PDF attachment URL. When disabled, it links to the Part-DB page
(old behavior). Configurable via settings UI or EDA_KICAD_DATASHEET_AS_PDF env var.
