💄 style(ui): replace inline gradients with reusable pastel blur balls; improve dark mode

- Introduce a global CSS utility `with-pastel-balls` in `web/src/index.css`, rendering pastel “blur balls” via ::before with CSS variables (`--pb1..--pb4`, `--pb-opacity`, `--pb-blur`) for easy theming.
- Apply the utility to pricing header cards and skeletons:
  - `web/src/components/table/model-pricing/layout/header/PricingVendorIntro.jsx`
  - `web/src/components/table/model-pricing/layout/header/PricingVendorIntroSkeleton.jsx`
- Remove per-component inline `linear-gradient(...)` backgrounds and redundant absolute-positioned decoration nodes to reduce duplication.
- Dark mode:
  - Keep the same pastel palette (pink/lavender/mint/peach).
  - Increase visibility with `--pb-opacity: 0.36`, `--pb-blur: 65px`, and `mix-blend-mode: screen`.
- No functional logic changes; UI-only. Lint passes.

Affected files:
- web/src/index.css
- web/src/components/table/model-pricing/layout/header/PricingVendorIntro.jsx
- web/src/components/table/model-pricing/layout/header/PricingVendorIntroSkeleton.jsx

web/src/components/table/model-pricing/layout/header/PricingVendorIntro.jsx

@@ -166,7 +166,7 @@ const PricingVendorIntro = ({
   if (filterVendor === 'all') {
     return (
       <div className='mb-4'>
-        <Card className="!rounded-2xl" bodyStyle={{ padding: '16px' }}>
+        <Card className="!rounded-2xl with-pastel-balls" bodyStyle={{ padding: '16px' }}>
           <div className="flex items-start space-x-3 md:space-x-4">
             {/* 全部供应商的头像组合 */}
             <div className="flex-shrink-0">
@@ -210,7 +210,7 @@ const PricingVendorIntro = ({
 
   return (
     <div className='mb-4'>
-      <Card className="!rounded-2xl" bodyStyle={{ padding: '16px' }}>
+      <Card className="!rounded-2xl with-pastel-balls" bodyStyle={{ padding: '16px' }}>
         <div className="flex items-start space-x-3 md:space-x-4">
           {/* 供应商图标 */}
           <div className="flex-shrink-0">

web/src/components/table/model-pricing/layout/header/PricingVendorIntroSkeleton.jsx

@@ -25,7 +25,7 @@ const PricingVendorIntroSkeleton = ({
 }) => {
   const placeholder = (
     <div className='mb-4'>
-      <Card className="!rounded-2xl" bodyStyle={{ padding: '16px' }}>
+      <Card className="!rounded-2xl with-pastel-balls" bodyStyle={{ padding: '16px' }}>
         <div className="flex items-start space-x-3 md:space-x-4">
           {/* 供应商图标骨架 */}
           <div className="flex-shrink-0 min-w-16 h-16 rounded-2xl bg-white shadow-md flex items-center justify-center px-2">