Add timezone backend support (handlers, utilities, user context)

- Add FormatInTimezone and FormatTimestamptzInTimezone helpers
  in templates/utils.go for timezone-aware time display
- Add Timezone field to templates.User struct
- Pass user timezone from DB to template context in helpers.go
- Add timezone update handling in auth.go UpdateProfile with
  validation via time.LoadLocation
- Add UpdateTimezoneSettings handler in system_settings.go for
  admin system-wide default timezone using UpdateSystemSetting

internal/router/helpers.go

@@ -35,6 +35,11 @@ func getTemplateUserWithTheme(c *echo.Context, cfg *Config) (templates.User, err
 		userTheme = userDB.Theme.String
 	}
 
+	userTimezone := "UTC"
+	if userDB.Timezone.Valid {
+		userTimezone = userDB.Timezone.String
+	}
+
 	// Extract JWT token for WebSocket authentication
 	token := ""
 	if cookie, err := c.Cookie("token"); err == nil {
@@ -48,6 +53,7 @@ func getTemplateUserWithTheme(c *echo.Context, cfg *Config) (templates.User, err
 		Role:     userRole,
 		Theme:    userTheme,
 		Token:    token,
+		Timezone: userTimezone,
 	}, nil
 }