refactor: unify docker deployment and restructure frontend architecture

This commit implements a unified Docker deployment strategy, moving from separate frontend and backend images to a single, multi-stage build image containing both services. It also introduces a major reorganization of the frontend directory structure and simplifies the environment configuration.

Key changes:
- **Deployment**: Added a multi-stage `Dockerfile` and `docker-entrypoint.sh` to package the Go backend and Nginx-served frontend into a single container.
- **CI/CD**: Updated GitHub Actions workflows (`ci-cd.yml`, `release.yml`) to build and push the new unified image instead of separate ones.
- **Frontend Refactor**: Reorganized `frontend/src/pages` into a domain-driven directory structure (e.g., `auth/`, `admin/`, `content/`, `communication/`, `productivity/`, `settings/`, `misc/`).
- **Configuration**: Simplified `.env.example` and updated `docker-compose.yml` to reflect the unified service model and single host port.
- **Cleanup**: Removed deprecated `docker-compose.demo.yml`, `docker-compose.prod.yml`, and various unused frontend components and services.
- **Backend**: Refactored configuration loading to use exported `GetDurationEnv` for better consistency.

.github/workflows/ci-cd.yml

@@ -128,43 +128,23 @@ jobs:
         password: ${{ secrets.GITHUB_TOKEN }}
 
     - name: Extract metadata
-      id: meta-backend
+      id: meta
       uses: docker/metadata-action@v4
       with:
-        images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}/backend
+        images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
         tags: |
           type=ref,event=branch
           type=ref,event=pr
           type=sha,prefix={{branch}}-
           type=raw,value=latest,enable={{is_default_branch}}
 
-    - name: Extract metadata
-      id: meta-frontend
-      uses: docker/metadata-action@v4
-      with:
-        images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}/frontend
-        tags: |
-          type=ref,event=branch
-          type=ref,event=pr
-          type=sha,prefix={{branch}}-
-          type=raw,value=latest,enable={{is_default_branch}}
-
-    - name: Build and push backend image
-      uses: docker/build-push-action@v4
-      with:
-        context: ./backend
-        push: true
-        tags: ${{ steps.meta-backend.outputs.tags }}
-        labels: ${{ steps.meta-backend.outputs.labels }}
-
-    - name: Build and push frontend image
+    - name: Build and push unified image
       uses: docker/build-push-action@v4
       with:
         context: .
-        file: ./frontend/Dockerfile
         push: true
-        tags: ${{ steps.meta-frontend.outputs.tags }}
-        labels: ${{ steps.meta-frontend.outputs.labels }}
+        tags: ${{ steps.meta.outputs.tags }}
+        labels: ${{ steps.meta.outputs.labels }}
 
   # deploy:
   #   name: Deploy to Production