feat(crop-rotate): keep aspect ratio orientation on rotate

[hm21]

Summary

Closes #655.

Adds an opt-in flag CropRotateEditorConfigs.enableKeepAspectRatioOnRotate that keeps the crop frame's orientation when the image is rotated. With it enabled, a 9:16 crop stays 9:16 after a 90° rotation instead of inverting to 16:9, and the image is zoomed in so it still fully covers the crop area.

ProImageEditorConfigs(
  cropRotateEditor: CropRotateEditorConfigs(
    enableKeepAspectRatioOnRotate: true,
  ),
)

• Works for fixed, original (0) and free (-1) aspect ratios — the current crop frame orientation is preserved by inverting its ratio.
• Defaults to false, so existing behavior is unchanged (backward compatible, as requested in the issue).

How it works

• The crop frame lives inside the rotating image transform, so letting it rotate would make a 9:16 frame visibly widen towards 1:1 at the 45° mark. To avoid this, during a locked rotation the crop overlay is faded out, the image rotates + zooms while the frame is hidden, and the frame is then faded back in at the preserved aspect ratio.
• The target crop is computed by inverting the current crop ratio (calcCropRect(newRatio: …)), which keeps the on-screen orientation regardless of the ratio mode.
• The rotation mechanics (angle, flipX/flipY, export cropRect) are unchanged, so cropping/export stay correct — no flip-swap changes were needed.

Changes

• CropRotateEditorConfigs: new enableKeepAspectRatioOnRotate flag (+ copyWith).
• CropRotateEditor: locked-rotation flow (fade out → rotate/zoom → fade in); only active when the flag is enabled, otherwise the normal rotate path is used.
• CropCornerPainter: new frameOpacity multiplier to cleanly fade the crop overlay to transparent (the existing fadeInOpacity blends towards the background/hero state, which would hide the image outside the crop while rotating).

Tests

Added widget tests covering the enabled (fixed + free ratio) and disabled cases. flutter analyze is clean and all crop-rotate tests pass.