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.

frontend/nginx.conf

@@ -55,9 +55,9 @@ http {
             try_files $uri $uri/ /index.html;
         }
 
-        # API proxy to backend with retry logic
+        # API proxy to backend (internal localhost)
         location /api/ {
-            proxy_pass http://trackeep-backend:8080/;
+            proxy_pass http://localhost:8080/;
             proxy_http_version 1.1;
             proxy_set_header Upgrade $http_upgrade;
             proxy_set_header Connection 'upgrade';
@@ -66,7 +66,7 @@ http {
             proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
             proxy_set_header X-Forwarded-Proto $scheme;
             proxy_cache_bypass $http_upgrade;
-            
+
             # Retry configuration
             proxy_connect_timeout 5s;
             proxy_send_timeout 10s;
@@ -74,7 +74,7 @@ http {
             proxy_next_upstream error timeout invalid_header http_500 http_502 http_503 http_504;
             proxy_next_upstream_tries 3;
             proxy_next_upstream_timeout 30s;
-            
+
             # Buffer settings
             proxy_buffering on;
             proxy_buffer_size 4k;