fix: improve structural and DPI validation

[Teresa1228]

Summary

• mark unimplemented structural checks as not automatically detected instead of showing false success
• validate the configured bleed against the selected material recommendation
• compute effective image DPI from cropped pixels on both axes and use the lower value
• add pure-function tests for crop, non-uniform scaling, and invalid dimensions

Verification

• npm test
• npm run check:template-sync
• npm run build
• git diff --check