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.

.github/workflows/docker-build.yml

@@ -3,22 +3,85 @@ name: Docker Build
 on:
   push:
     branches: [main]
+    tags: ["v*"]
   pull_request:
     branches: [main]
 
+env:
+  REGISTRY: ghcr.io
+  IMAGE_PREFIX: ${{ github.repository }}
+
 jobs:
   build-backend:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
 
-      - name: Build backend image
-        run: docker build -f backend/Dockerfile .
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Log in to registry (push only)
+        if: github.event_name == 'push'
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Extract metadata
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: ${{ env.REGISTRY }}/${{ env.IMAGE_PREFIX }}/backend
+          tags: |
+            type=ref,event=branch
+            type=semver,pattern={{version}}
+            type=sha
+
+      - name: Build and push backend image
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          file: backend/Dockerfile
+          push: ${{ github.event_name == 'push' }}
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max
 
   build-frontend:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
 
-      - name: Build frontend image
-        run: docker build -f frontend/Dockerfile ./frontend
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Log in to registry (push only)
+        if: github.event_name == 'push'
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Extract metadata
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: ${{ env.REGISTRY }}/${{ env.IMAGE_PREFIX }}/frontend
+          tags: |
+            type=ref,event=branch
+            type=semver,pattern={{version}}
+            type=sha
+
+      - name: Build and push frontend image
+        uses: docker/build-push-action@v6
+        with:
+          context: ./frontend
+          file: frontend/Dockerfile
+          push: ${{ github.event_name == 'push' }}
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max