feat(standard-styling) Enhance search and styling

Improves search bar with SVG icon support for better visual consistency.

Adds utility SCSS file for managing global styles like heading selectors.

Introduces a checkmark SVG asset and reorganizes custom SCSS files following the 7-in-1 structure for better maintainability.

Comments out optional USP bar code in header.tpl to allow easy activation.

falcon/_dev/css/theme/custom/abstracts/_utilities.scss

@@ -0,0 +1,3 @@
+// Header selector for interpolation
+// Example usage: #{$headings} { ... }
+$headings: "h1, h2, h3, h4, h5, h6, .h1, .h2, .h3, .h4, .h5, .h6";