style: restore original color scheme to shadcn/ui components

Restore the vibrant color palette from the pre-refactoring version while
maintaining shadcn/ui component architecture and modern design patterns.

## Color Scheme Restoration

### Button Component
- **default variant**: Blue primary (`bg-blue-500`) - matches old `primary`
- **destructive variant**: Red (`bg-red-500`) - matches old `danger`
- **secondary variant**: Gray text (`text-gray-500`) - matches old `secondary`
- **ghost variant**: Transparent hover (`hover:bg-gray-100`) - matches old `ghost`
- **mcp variant**: Emerald green (`bg-emerald-500`) - matches old `mcp`
- Updated border-radius to `rounded-lg` for consistency

### CSS Variables
- Set `--primary` to blue (`hsl(217 91% 60%)` ≈ `bg-blue-500`)
- Added complete shadcn/ui theme variables for light/dark modes
- Maintained semantic color tokens for maintainability

### Component-Specific Colors
- **"Currently Using" badge**: Green (`bg-green-500/10 text-green-500`)
- **Delete button hover**: Red (`hover:text-red-500 hover:bg-red-100`)
- **MCP button**: Emerald green with minimum width (`min-w-[80px]`)
- **Links/URLs**: Blue (`text-blue-500`)

## Benefits

- ✅ Restored original vibrant UI (blue, green, red accents)
- ✅ Maintained shadcn/ui component system (accessibility, animations)
- ✅ Easy global theming via CSS variables
- ✅ Consistent design language across all components
- ✅ Code formatted with Prettier (shadcn/ui standards)

## Files Changed

- `src/index.css`: Added shadcn/ui theme variables with blue primary
- `src/components/ui/button.tsx`: Restored all original button color variants
- `src/components/providers/ProviderCard.tsx`: Green badge for current provider
- `src/components/providers/ProviderActions.tsx`: Red hover for delete button
- `src/components/mcp/McpPanel.tsx`: Use `mcp` variant for consistency
- `src/App.tsx`: MCP button with emerald color and wider width

The UI now matches the original colorful design while leveraging modern
shadcn/ui components for better maintainability and user experience.

src/components/ui/button.tsx

@@ -4,26 +4,35 @@ import { cva, type VariantProps } from "class-variance-authority";
 import { cn } from "@/lib/utils";
 
 const buttonVariants = cva(
-  "inline-flex items-center justify-center gap-2 whitespace-nowrap rounded-md text-sm font-medium transition-colors focus-visible:outline-none focus-visible:ring-1 focus-visible:ring-ring disabled:pointer-events-none disabled:opacity-50",
+  "inline-flex items-center justify-center gap-2 whitespace-nowrap rounded-lg text-sm font-medium transition-colors focus-visible:outline-none focus-visible:ring-1 focus-visible:ring-ring disabled:pointer-events-none disabled:opacity-50",
   {
     variants: {
       variant: {
+        // 主按钮：蓝底白字（对应旧版 primary）
         default:
-          "bg-primary text-primary-foreground shadow hover:bg-primary/90",
+          "bg-blue-500 text-white hover:bg-blue-600 dark:bg-blue-600 dark:hover:bg-blue-700",
+        // 危险按钮：红底白字（对应旧版 danger）
         destructive:
-          "bg-destructive text-destructive-foreground shadow-sm hover:bg-destructive/90",
+          "bg-red-500 text-white hover:bg-red-600 dark:bg-red-600 dark:hover:bg-red-700",
+        // 轮廓按钮
         outline:
-          "border border-input bg-background shadow-sm hover:bg-accent hover:text-accent-foreground",
+          "border border-gray-200 bg-background hover:bg-gray-100 dark:border-gray-700 dark:hover:bg-gray-800",
+        // 次按钮：灰色（对应旧版 secondary）
         secondary:
-          "bg-secondary text-secondary-foreground shadow-sm hover:bg-secondary/80",
-        ghost: "hover:bg-accent hover:text-accent-foreground",
-        link: "text-primary underline-offset-4 hover:underline",
+          "text-gray-500 hover:bg-gray-100 dark:text-gray-400 dark:hover:bg-gray-800 dark:hover:text-gray-200",
+        // 幽灵按钮（对应旧版 ghost）
+        ghost:
+          "text-gray-500 hover:text-gray-900 hover:bg-gray-100 dark:text-gray-400 dark:hover:text-gray-100 dark:hover:bg-gray-800",
+        // MCP 专属按钮：祖母绿
+        mcp: "bg-emerald-500 text-white hover:bg-emerald-600 dark:bg-emerald-600 dark:hover:bg-emerald-700",
+        // 链接按钮
+        link: "text-blue-500 underline-offset-4 hover:underline dark:text-blue-400",
       },
       size: {
         default: "h-9 px-4 py-2",
         sm: "h-8 rounded-md px-3 text-xs",
         lg: "h-10 rounded-md px-8",
-        icon: "h-9 w-9",
+        icon: "h-9 w-9 p-1.5",
       },
     },
     defaultVariants: {