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/ThemeSearch.php

@@ -388,12 +388,10 @@ class ThemeSearch {
         return '';
     }
 
-    private const HEX_PATTERN = '/^#([0-9a-fA-F]{3}|[0-9a-fA-F]{6})$/';
-
     /** 全局主题色：theme_skin_custom 优先，清空时使用 theme_skin 预置色（对齐参考主题 zib_head_css）。 */
     public static function theme_color(): string {
-        $custom = trim((string) Settings::get_value('theme_skin_custom', ''));
-        if (preg_match(self::HEX_PATTERN, $custom)) {
+        $custom = \AKSRT\Core\Color::sanitize(Settings::get_value('theme_skin_custom', ''));
+        if ($custom !== '') {
             return $custom;
         }
         $skin = ltrim(trim((string) Settings::get_value('theme_skin', 'f04494')), '#');
@@ -402,8 +400,7 @@ class ThemeSearch {
 
     /** 夜间模式主题色；未设置返回空串，夜间沿用全局主题色。 */
     public static function dark_theme_color(): string {
-        $color = trim((string) Settings::get_value('dark_theme_skin_custom', ''));
-        return preg_match(self::HEX_PATTERN, $color) ? $color : '';
+        return \AKSRT\Core\Color::sanitize(Settings::get_value('dark_theme_skin_custom', ''));
     }
 
     public static function get_theme_mode_button_positions(): array {
@@ -429,8 +426,7 @@ class ThemeSearch {
         if (!is_array($bg)) {
             return '';
         }
-        $color = trim((string) ($bg['color'] ?? ''));
-        $color = preg_match(self::HEX_PATTERN, $color) ? $color : '';
+        $color = \AKSRT\Core\Color::sanitize($bg['color'] ?? '');
         $image = esc_url_raw(trim((string) ($bg['image'] ?? '')));
         if ($color === '' && $image === '') {
             return '';