Design QA | Spacing QA mode (catalog + highlight)

[sm17p]

Part of epic #59.

Scope

• Spacing QA mode in the side panel: viewport-scoped scan, catalog of margin/padding values with counts, toggles for margins vs padding, select row to highlight all matching elements, Clear.
• Collector using getComputedStyle; QA highlight layer consistent with existing overlays.

Spec

Product scope and acceptance: epic #59.

Depends on

• Side panel shell / tabs (Add sidepanel  #13)
• Resize (Handle viewport resize #7), scroll (Update tracker position on scroll #12), shortcuts (Shortcuts buggy when typing onscreen #14) for hardening