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.

docker-entrypoint.sh

@@ -0,0 +1,41 @@
+#!/bin/sh
+
+# Unified entrypoint for Trackeep
+# Starts both backend and nginx in one container
+
+set -e
+
+# Backend configuration
+export BACKEND_PORT=${BACKEND_PORT:-8080}
+export DB_HOST=${DB_HOST:-postgres}
+export DB_PORT=${DB_PORT:-5432}
+export DB_NAME=${DB_NAME:-trackeep}
+export DB_USER=${DB_USER:-trackeep}
+export DB_PASSWORD=${DB_PASSWORD}
+export DRAGONFLY_ADDR=${DRAGONFLY_ADDR:-dragonfly:6379}
+export DRAGONFLY_PASSWORD=${DRAGONFLY_PASSWORD}
+export JWT_SECRET=${JWT_SECRET}
+export GIN_MODE=${GIN_MODE:-release}
+
+# Start backend in background
+cd /app
+echo "Starting Trackeep backend on port ${BACKEND_PORT}..."
+./main &
+
+# Wait for backend to be ready
+echo "Waiting for backend to be ready..."
+for i in $(seq 1 30); do
+    if wget --no-verbose --tries=1 --spider http://localhost:${BACKEND_PORT}/health 2>/dev/null; then
+        echo "Backend is ready!"
+        break
+    fi
+    echo "Waiting... ($i/30)"
+    sleep 2
+done
+
+# Update nginx config to proxy to localhost backend
+sed -i "s|http://trackeep-backend:8080/|http://localhost:${BACKEND_PORT}/|g" /etc/nginx/nginx.conf
+
+# Start nginx
+echo "Starting nginx..."
+nginx -g "daemon off;"