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>

src/Admin/PageConfigMetaBox.php

@@ -122,7 +122,7 @@ final class PageConfigMetaBox {
 		$config = [
 			'header_slider_show'          => !empty($raw['header_slider_show']),
 			'header_slider_show_type'     => self::enum((string) ($raw['header_slider_show_type'] ?? ''), ['', 'only_pc', 'only_sm']),
-			'header_slider_nav_color'     => sanitize_hex_color((string) ($raw['header_slider_nav_color'] ?? '')) ?: '#ffffff',
+			'header_slider_nav_color'     => \AKSRT\Core\Color::sanitize((string) ($raw['header_slider_nav_color'] ?? '')) ?: '#ffffff',
 			'header_slider'               => self::sanitize_slides(self::decode_json($raw['header_slider'] ?? '')),
 			'header_slider_option'        => self::sanitize_slider_options(self::decode_json($raw['header_slider_option'] ?? '')),
 			'header_slider_search_s'      => self::sanitize_devices($raw['header_slider_search_s'] ?? []),