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/Settings/DisplayPageGroups.php

@@ -77,7 +77,7 @@ trait DisplayPageGroups {
 			'slides'          => is_array($s['header_slider'] ?? null) ? $s['header_slider'] : self::header_slider_default_slides(),
 			'show_on_pages'   => $show_on,
 			'show_type'       => $show_type,
-			'nav_color'       => sanitize_hex_color((string) ($s['header_slider_nav_color'] ?? '')) ?: '',
+			'nav_color'       => \AKSRT\Core\Color::sanitize((string) ($s['header_slider_nav_color'] ?? '')) ?: '',
 			'slider_config'   => $slider_option,
 			'search_show'     => self::sanitize_device_visibility($s['header_slider_search_s'] ?? ['pc_s', 'm_s']),
 			'search_config'   => $search_option,