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/FloatButtons.php

@@ -218,25 +218,8 @@ class FloatButtons {
 	private static function sanitize_color_group($group): array {
 		$group = is_array($group) ? $group : [];
 		return [
-			'color' => self::sanitize_color((string) ($group['color'] ?? '')),
-			'bg'    => self::sanitize_color((string) ($group['bg'] ?? '')),
+			'color' => \AKSRT\Core\Color::sanitize($group['color'] ?? '', true),
+			'bg'    => \AKSRT\Core\Color::sanitize($group['bg'] ?? '', true),
 		];
 	}
-
-	/**
-	 * 允许 #hex（3/4/6/8 位）与 rgb()/rgba()。
-	 */
-	public static function sanitize_color(string $value): string {
-		$value = trim($value);
-		if ($value === '') {
-			return '';
-		}
-		if (preg_match('/^#(?:[0-9a-fA-F]{3,4}|[0-9a-fA-F]{6}|[0-9a-fA-F]{8})$/', $value)) {
-			return $value;
-		}
-		if (preg_match('/^rgba?\(\s*\d{1,3}\s*,\s*\d{1,3}\s*,\s*\d{1,3}\s*(?:,\s*(?:0|1|0?\.\d+)\s*)?\)$/', $value)) {
-			return $value;
-		}
-		return '';
-	}
 }