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.

backend/handlers/auth.go

@@ -11,7 +11,6 @@ import (
 	"os"
 	"os/exec"
 	"runtime"
-	"strconv"
 	"strings"
 	"time"
 
@@ -68,25 +67,6 @@ type Claims struct {
 	jwt.RegisteredClaims
 }
 
-// getDurationEnv parses duration from environment variable with fallback
-func getDurationEnv(key string, defaultValue time.Duration) time.Duration {
-	value := os.Getenv(key)
-	if value == "" {
-		return defaultValue
-	}
-
-	seconds, err := strconv.Atoi(value)
-	if err != nil {
-		duration, err := time.ParseDuration(value)
-		if err != nil {
-			return defaultValue
-		}
-		return duration
-	}
-
-	return time.Duration(seconds) * time.Second
-}
-
 // GenerateJWT creates a new JWT token for a user
 func GenerateJWT(user models.User) (string, error) {
 	return generateJWT(user)
@@ -103,7 +83,7 @@ func generateJWT(user models.User) (string, error) {
 		Username: user.Username,
 		GitHubID: user.GitHubID,
 		RegisteredClaims: jwt.RegisteredClaims{
-			ExpiresAt: jwt.NewNumericDate(time.Now().Add(getDurationEnv("JWT_EXPIRES_IN", 24*time.Hour))),
+			ExpiresAt: jwt.NewNumericDate(time.Now().Add(config.GetDurationEnv("JWT_EXPIRES_IN", 24*time.Hour))),
 			IssuedAt:  jwt.NewNumericDate(time.Now()),
 			Issuer:    "trackeep",
 		},