fix: optimize CSS color variables for WCAG AA compliance

- Changed --color-bg-primary from #FAFAFA to #FFFFFF
- Changed --color-text-tertiary from #5C5C5C to #4A4A4A
- Changed --color-text-muted from #8C8C8C to #6B6B6B
- All color combinations now meet WCAG AA 4.5:1 contrast ratio

src/app/globals.css

@@ -31,7 +31,7 @@
   --color-brand-primary-bg: #FEF2F4;
   
   /* 背景色系 - 宣纸白 */
-  --color-bg-primary: #FAFAFA;
+  --color-bg-primary: #FFFFFF;
   --color-bg-secondary: #FFFBF5;
   --color-bg-tertiary: #F5F5F5;
   --color-bg-hover: #EFEFEF;
@@ -39,8 +39,8 @@
   /* 文字色系 - 墨色层次 */
   --color-text-primary: #1C1C1C;
   --color-text-secondary: #3D3D3D;
-  --color-text-tertiary: #5C5C5C;
-  --color-text-muted: #8C8C8C;
+  --color-text-tertiary: #4A4A4A;
+  --color-text-muted: #6B6B6B;
   
   /* 边框色系 */
   --color-border-primary: #E5E5E5;