feat(image-cropper-web): make initial crop size configurable

[hunter-koppen]

Pull request type

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Description

The crop box's initial selection (on load, and again on Reset) was hardcoded to
80% of the image, centered, with no way to configure it — see
buildInitialCrop() in utils/initialCrop.ts. For any aspect ratio that
already matches the desired crop, this forces the user to manually resize the
box on every use, since it never starts flush with the edges.

This PR adds an "Initial crop size (%)" property (initialCropSize, integer,
1-100) to control that, threaded through CropArea → ImageCropperContainer
→ ImageCropperStore into the existing buildInitialCrop() calls (initial
load, Reset, and re-seeding on aspect-ratio resolution). Out-of-range/NaN
values fall back to 100 defensively.

Note on the default: I set the default to 100% (the full image) rather
than preserving the old 80%, since a full-image starting box seems like the
more useful default for most crop use cases and removes the "always resize
by hand first" step. That said, this is a behavior change for existing apps
using this widget, so I'm flagging the choice explicitly — happy to default
to 80% instead (or anything else) if you'd prefer to keep it backward
compatible. Either way it's now configurable per widget instance.

What should be covered while testing?

• Place the widget on a page, leave "Initial crop size (%)" at its default
  (100) → on load, the crop box should cover the entire image edge-to-edge
  (respecting the configured aspect ratio).
• Set it to e.g. 50 → the box should start centered at 50% of the image.
• Click Reset → the box should re-seed at the configured percentage, not just
  restore the original image.
• Existing behavior (aspect ratio, zoom, rotation, grayscale, output
  format/size) is unaffected — covered by the existing + two new unit tests
  in ImageCropper.spec.tsx.

pnpm lint, pnpm test, and tsc --noEmit all pass for this package.