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/shell/header.tsx

@@ -25,7 +25,7 @@ export function Header({
   const dateStr = now.toLocaleDateString([], { weekday: "short", month: "short", day: "numeric" });
 
   return (
-    <header className="sticky top-0 z-40 w-full border-b border-border bg-background">
+    <header className="sticky top-0 z-40 w-full border-b border-border bg-card">
       <div className="mx-auto flex h-14 max-w-6xl items-center justify-between px-4">
         <div className="flex items-center gap-4">
           <div className="flex items-center gap-2">