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/Core/Color.php

@@ -0,0 +1,55 @@
+<?php
+/**
+ * 全站唯一的颜色值净化器（SPA 设置、小工具、悬浮按钮、工单状态、前台渲染共用）。
+ *
+ * 前端对应实现：assets/admin/src/utils/color.js（normalizeColor），两端规则保持一致。
+ *
+ * @package AKSRT
+ */
+
+namespace AKSRT\Core;
+
+defined('ABSPATH') || exit;
+
+final class Color {
+
+	private const HEX = '/^#?([0-9a-f]{3}|[0-9a-f]{6})$/i';
+	private const HEX_ALPHA = '/^#?([0-9a-f]{3,4}|[0-9a-f]{6}|[0-9a-f]{8})$/i';
+	private const RGB = '/^rgba?\(\s*(\d{1,3})\s*,\s*(\d{1,3})\s*,\s*(\d{1,3})\s*(?:,\s*(0(?:\.\d+)?|1(?:\.0+)?|\.\d+)\s*)?\)$/i';
+
+	/**
+	 * 返回规范化颜色；非法值返回空串。
+	 *
+	 * - 默认只接受 #rgb / #rrggbb（可省略 #），输出小写并补 #。
+	 * - $allow_alpha 时额外接受 #rgba / #rrggbbaa 与 rgb()/rgba()，
+	 *   rgb 形态统一输出为 `rgb(r, g, b)` / `rgba(r, g, b, a)`。
+	 *
+	 * @param mixed $value
+	 */
+	public static function sanitize($value, bool $allow_alpha = false): string {
+		if (!is_string($value)) {
+			return '';
+		}
+		$value = trim($value);
+		if ($value === '') {
+			return '';
+		}
+		if (preg_match($allow_alpha ? self::HEX_ALPHA : self::HEX, $value, $m) === 1) {
+			return '#' . strtolower($m[1]);
+		}
+		if (!$allow_alpha || preg_match(self::RGB, $value, $m) !== 1) {
+			return '';
+		}
+		$channels = [(int) $m[1], (int) $m[2], (int) $m[3]];
+		foreach ($channels as $channel) {
+			if ($channel > 255) {
+				return '';
+			}
+		}
+		if (!isset($m[4]) || $m[4] === '') {
+			return 'rgb(' . implode(', ', $channels) . ')';
+		}
+		$alpha = rtrim(rtrim(number_format((float) $m[4], 3, '.', ''), '0'), '.');
+		return 'rgba(' . implode(', ', $channels) . ', ' . ($alpha === '' ? '0' : $alpha) . ')';
+	}
+}