fix(ui): improve ProviderIcon color validation to prevent black icons

Enhance the effectiveColor logic in ProviderIcon to properly validate
the color prop before using it. Now only uses color when it's a valid
non-empty string, otherwise falls back to icon metadata defaultColor.

This fixes the issue where Gemini icons would turn black when selected
in ProviderCard due to null/undefined/empty string color values being
passed through.

src/components/ProviderIcon.tsx

@@ -39,9 +39,13 @@ export const ProviderIcon: React.FC<ProviderIconProps> = ({
     };
   }, [size]);
 
-  // 获取有效颜色：优先使用传入的 color，否则从元数据获取 defaultColor
+  // 获取有效颜色：优先使用传入的有效 color，否则从元数据获取 defaultColor
   const effectiveColor = useMemo(() => {
-    if (color) return color;
+    // 只有当 color 是有效的非空字符串时才使用
+    if (color && typeof color === 'string' && color.trim() !== '') {
+      return color;
+    }
+    // 否则从元数据获取 defaultColor
     if (icon) {
       const metadata = getIconMetadata(icon);
       // 只有当 defaultColor 不是 currentColor 时才使用