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/index.css

@@ -1,12 +1,79 @@
 /* 引入 Tailwind v4 内建样式与工具 */
 @import "tailwindcss";
 
-/* 覆盖 Tailwind v4 默认的 dark 变体为“类选择器”模式 */
+/* 覆盖 Tailwind v4 默认的 dark 变体为"类选择器"模式 */
 @custom-variant dark (&:where(.dark, .dark *));
 
+/* shadcn/ui 主题变量 - 蓝色主题 */
+@layer base {
+  :root {
+    --background: 0 0% 100%;
+    --foreground: 240 10% 3.9%;
+    --card: 0 0% 100%;
+    --card-foreground: 240 10% 3.9%;
+    --popover: 0 0% 100%;
+    --popover-foreground: 240 10% 3.9%;
+
+    /* 主色调：蓝色 (bg-blue-500) */
+    --primary: 217 91% 60%;
+    --primary-foreground: 0 0% 100%;
+
+    /* 次要色：淡蓝灰 */
+    --secondary: 240 4.8% 95.9%;
+    --secondary-foreground: 240 5.9% 10%;
+
+    /* 强调色 */
+    --muted: 240 4.8% 95.9%;
+    --muted-foreground: 240 3.8% 46.1%;
+    --accent: 240 4.8% 95.9%;
+    --accent-foreground: 240 5.9% 10%;
+
+    /* 危险色：红色 */
+    --destructive: 0 84.2% 60.2%;
+    --destructive-foreground: 0 0% 98%;
+
+    /* 边框和输入框 */
+    --border: 240 5.9% 90%;
+    --input: 240 5.9% 90%;
+    --ring: 217 91% 60%;
+
+    --radius: 0.5rem;
+  }
+
+  .dark {
+    --background: 240 10% 3.9%;
+    --foreground: 0 0% 98%;
+    --card: 240 10% 3.9%;
+    --card-foreground: 0 0% 98%;
+    --popover: 240 10% 3.9%;
+    --popover-foreground: 0 0% 98%;
+
+    /* 暗色模式主色调：稍深的蓝色 (bg-blue-600) */
+    --primary: 217 91% 55%;
+    --primary-foreground: 0 0% 100%;
+
+    --secondary: 240 3.7% 15.9%;
+    --secondary-foreground: 0 0% 98%;
+
+    --muted: 240 3.7% 15.9%;
+    --muted-foreground: 240 5% 64.9%;
+    --accent: 240 3.7% 15.9%;
+    --accent-foreground: 0 0% 98%;
+
+    /* 暗色模式危险色 */
+    --destructive: 0 62.8% 30.6%;
+    --destructive-foreground: 0 0% 98%;
+
+    --border: 240 3.7% 15.9%;
+    --input: 240 3.7% 15.9%;
+    --ring: 217 91% 55%;
+  }
+}
+
 /* 全局基础样式 */
 * {
   box-sizing: border-box;
+  border-color: hsl(var(--border));
 }
 
 html {
@@ -17,7 +84,9 @@ html {
 }
 
 body {
-  @apply m-0 p-0 bg-gray-50 text-gray-900 text-sm dark:bg-gray-950 dark:text-gray-100;
+  @apply m-0 p-0 text-sm;
+  background-color: hsl(var(--background));
+  color: hsl(var(--foreground));
 }
 
 /* 暗色模式下启用暗色原生控件/滚动条配色 */