ci: update docker build workflow and refine frontend theme

Refactor the CI/CD pipeline to use Docker Buildx for more efficient builds and implement automated image tagging and pushing to GHCR.

On the frontend, update the theme system to use a neutral zinc-based dark mode instead of the previous warm dark theme. This includes:
- Updating CSS variables in `globals.css` for a more consistent neutral palette.
- Replacing `ring` color usage with `muted-foreground` in various UI components to align with the new design language.
- Adjusting component backgrounds (e.g., `Header`, `Input`, `WidgetCard`) to use `bg-card` for better visual layering.
- Simplifying component styles and removing unnecessary gradients.

frontend/components/services/service-card.tsx

@@ -157,8 +157,8 @@ export function ServiceCard({
         )}
         onClick={handleClick}
       >
-        {/* Gradient accent line at top */}
-        <div className="absolute top-0 left-0 right-0 h-[2px] opacity-0 group-hover:opacity-100 transition-opacity bg-ring" />
+        {/* Accent line at top */}
+        <div className="absolute top-0 left-0 right-0 h-[2px] opacity-0 group-hover:opacity-100 transition-opacity bg-muted-foreground" />
 
         <div className="flex h-full flex-col items-center justify-center gap-2.5 p-4">