refactor(color): unify color sanitizing and picking behind shared components

- Add AKSRT\Core\Color::sanitize($value, $allow_alpha) as the single PHP
  color sanitizer; replace sanitize_hex_color calls and per-class regexes
  (FloatButtons, FieldSchema, ThemeSearch, icon cards, TicketStatusService)
- SchemaProvider / ControllerBase sanitize `color` fields as colors;
  alpha kept only when the schema sets showAlpha
- Split SchemaProvider OAuth validation and option matching into sibling
  traits to stay under the 1000-line limit
- Add AksColorPicker.vue + utils/color.js mirroring the PHP rules; AksField,
  FloatBtnCommonFields and TicketStatusPanel use it
- Widget background color input now uses wpColorPicker
- Remove dead AksCompositeField dimensions/spacing/background branches and
  the unused Gutenberg ColorPicker var
- Add color-unification contract test; update harness requires and tests

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

inc/helpers/homepage/header-components.php

@@ -443,7 +443,7 @@ if (!function_exists('aksrt_header_slide')) {
 			}
 		}
 
-		$nav_color = sanitize_hex_color((string) ($args['header_slider_nav_color'] ?? ''));
+		$nav_color = \AKSRT\Core\Color::sanitize((string) ($args['header_slider_nav_color'] ?? ''));
 		if ($nav_color && $nav_color !== '#ffffff') {
 			echo '<style>body.nav-fixed:not(.body-scroll):not(.mobile-navbar-show) .header.show-slide{--header-color:' . esc_html($nav_color) . ';}</style>';
 		}