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/App.tsx

@@ -249,7 +249,11 @@ function App() {
 
           <div className="flex flex-wrap items-center gap-3">
             <AppSwitcher activeApp={activeApp} onSwitch={setActiveApp} />
-            <Button variant="outline" onClick={() => setIsMcpOpen(true)}>
+            <Button
+              variant="mcp"
+              onClick={() => setIsMcpOpen(true)}
+              className="min-w-[80px]"
+            >
               MCP
             </Button>
             <Button onClick={() => setIsAddOpen(true)}>