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/groups/group-section.tsx

@@ -48,8 +48,8 @@ export function GroupSection({ group, onEditService, onDeleteService, onEditGrou
               className="flex flex-1 items-center gap-2.5 group/title min-w-0"
               onClick={handleToggle}
             >
-              <div className="flex h-7 w-7 items-center justify-center rounded-lg transition-colors bg-accent">
-                <FolderOpen className="h-3.5 w-3.5 text-accent-foreground" />
+              <div className="flex h-7 w-7 items-center justify-center rounded-lg transition-colors bg-secondary">
+                <FolderOpen className="h-3.5 w-3.5 text-secondary-foreground" />
               </div>
               <div className="flex items-center gap-2 min-w-0">
                 <span className="text-sm font-semibold truncate">{group.name}</span>