Refactor grid overlay to use DOM-based columns instead of background gradients for more accurate layout alignment.

• Major improvement: Refactored the frontend grid overlay to use real DOM-based columns instead of a repeating background gradient.
• The new DOM-based method improves alignment accuracy by adjusting for scrollbar width differences (using corrected 100vw calculations).
• This change resolves longstanding inconsistencies when aligning layouts against the grid across different browsers and screen sizes.