Revert the annotation card shadow to its pre-0.30.1 logic

The "fix: Corner Radii" work that shipped in 0.30.1 rewrote the preview's
shadow caster and coupled canvas layout to the shadow's blur kernel. Both
changed behaviour users could see:

- Padding became max(padding, shadowOutsets), so past a threshold the Shadow
  slider inflated the canvas. The card shrank, and because the kernel is
  asymmetric (bottom = blur + yOffset, top = blur - yOffset) it also drifted
  upward. Shadow was effectively a second Padding control.
- The caster switched from an opaque fill plus destinationOut knockout to
  shadowOnly with no knockout. A shadow is a blurred filled rounded rect,
  opaque through the middle, so the card interior was painted black again and
  a translucent border sat on top of it. Moving Shadow visibly changed how
  heavy the Border looked.

Both files go back to the versions from the commit before that change, which
is the rendering that shipped in 0.30.0. The export-side corner radii work
from the same commit is kept: it does not touch the preview.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>