Visual Comparison improvements

The Visual Compare step has been enhanced, giving you more control over how your UI is validated against reference designs.

• Full Page Audit mode: compare an uploaded reference image against the entire scrollable length of the live page, catching differences even below the fold
• Visible Viewport mode: compare the reference image against only what is currently visible in the browser window, useful for validating specific sections of a page
• Test results now clearly indicate Visual match or Visual mismatch with detailed reporting on any differences found

Sprint #25