Add four-corner perspective transforms

[SLP-DEV1]

Summary

Adds real four-corner perspective warping for flat image mockups.

Changes

• add optional perspective settings to mockup presets/settings
• add projective homography renderer for browser Canvas export
• reject self-intersecting and degenerate quadrilaterals
• add four draggable corner handles directly in the visual editor
• add enable/disable/reset controls for perspective mode
• make editor preview and batch export use the same perspective geometry
• keep existing presets backward compatible because perspective is optional
• add focused unit tests for homography mapping and invalid corner shapes

Closes #18