session: parse DesktopNames for XDG_CURRENT_DESKTOP, wire utmpx, fix shell

- .desktop files now parse DesktopNames: first ;-entry → XDG_SESSION_DESKTOP,
  full list with : separators → XDG_CURRENT_DESKTOP
- AddUtmpx/Close wired into spawnSession (logins appear in who/w/last)
- Credentials.Shell reads from /etc/passwd instead of hardcoded /bin/sh
- Added SetDesktopVars to env.go

src/session/scan.go

@@ -13,10 +13,11 @@ import (
 )
 
 type DesktopEntry struct {
-	Name      string
-	Exec      string
-	Hidden    bool
-	NoDisplay bool
+	Name         string
+	Exec         string
+	DesktopNames string
+	Hidden       bool
+	NoDisplay    bool
 }
 
 func parseDesktopEntry(path string) (*DesktopEntry, error) {
@@ -51,6 +52,8 @@ func parseDesktopEntry(path string) (*DesktopEntry, error) {
 			e.Name = v
 		case "Exec":
 			e.Exec = v
+		case "DesktopNames":
+			e.DesktopNames = v
 		case "Hidden":
 			e.Hidden = strings.EqualFold(v, "true")
 		case "NoDisplay":
@@ -97,15 +100,28 @@ func scanDesktopDir(dir, kind string, envs *[]PostLoginEnv) {
 		if err != nil || de.Hidden || de.NoDisplay {
 			continue
 		}
+		desktopNames, sessionDesktop := splitDesktopNames(de.DesktopNames, e.Name())
 		*envs = append(*envs, PostLoginEnv{
-			Kind:        kind,
-			Title:       de.Name,
-			XinitrcPath: de.Exec,
-			ScriptPath:  de.Exec,
+			Kind:           kind,
+			Title:          de.Name,
+			XinitrcPath:    de.Exec,
+			ScriptPath:     de.Exec,
+			DesktopNames:   desktopNames,
+			SessionDesktop: sessionDesktop,
 		})
 	}
 }
 
+func splitDesktopNames(raw, fallback string) (currentDesktop, sessionDesktop string) {
+	if raw == "" {
+		return fallback, fallback
+	}
+	parts := strings.SplitN(raw, ";", 2)
+	sessionDesktop = strings.TrimSpace(parts[0])
+	currentDesktop = strings.ReplaceAll(raw, ";", ":")
+	return
+}
+
 func scanScriptDir(dir, kind string, envs *[]PostLoginEnv) {
 	entries, err := os.ReadDir(dir)
 	if err != nil {