session: use systemctl for power verbs, not loginctl

loginctl manages sessions/users/seats only — `loginctl poweroff` exits 1
"Unknown command verb". Preferring it silently broke poweroff/reboot/
suspend/hibernate from every shell surface. Use systemctl; fall back to
loginctl only on elogind systems where it does carry power verbs. The
logind D-Bus capability probe (CanPowerOff etc.) is unchanged — logind
owns policy, systemctl carries out the action.

surfaces/quickshell/modules/common/functions/Session.qml

@@ -170,12 +170,21 @@ Singleton {
     // caller can distinguish "started" from "succeeded".
     property var lastAction: ({ action: "", status: "idle", exitCode: null })
 
-    // Prefer logind (owns the session, works under elogind) and fall back to
+    // systemctl owns the power verbs, NOT loginctl. loginctl only manages
-    // systemctl. Returns [] when neither exists, which callers treat as a
+    // sessions/users/seats — `loginctl poweroff` exits 1 with "Unknown
-    // refusal rather than firing a command that cannot work.
+    // command verb" (verified on systemd 261, and its --help lists no power
+    // commands at all). Preferring loginctl here silently broke poweroff,
+    // reboot, suspend and hibernate from every shell surface: the button
+    // fired, the Process exited 1, and the phone stayed on (2026-07-20).
+    //
+    // The capability probe still asks logind over D-Bus (CanPowerOff etc.) —
+    // that part was always right; logind owns the *policy*. It just isn't
+    // the CLI that carries out the action.
     function powerCommand(action) {
-        if (root.hasLoginctl) return ["loginctl", action];
         if (root.hasSystemctl) return ["systemctl", action];
+        // elogind ships loginctl with power verbs and usually no systemctl;
+        // only reachable on such a system, where these verbs do exist.
+        if (root.hasLoginctl) return ["loginctl", action];
         return [];
     }