fix(updater): run the handoff script via systemd-run so it survives the app's scope

On uwsm/Hyprland the app runs in a systemd scope cgroup; a detached child is
swept away when the app quits, which killed the swap mid-flight on every
in-app update attempt. systemd-run gives the installer its own transient
unit that outlives the scope; detached spawn stays as the non-systemd
fallback.

src/main/index.ts

@@ -1286,12 +1286,40 @@ handleIpc("folio:performUpdate", async () => {
       }),
       { mode: 0o700 }
     );
+    // IMPORTANT: on systemd desktops (Hyprland/uwsm) the app runs inside a
+    // systemd scope cgroup, and a plain detached child stays in that cgroup —
+    // when the app quits, systemd sweeps the scope and kills the swap script
+    // mid-run (root cause of the "update failed" reports: the handoff died
+    // before swapping, and the app was relaunched from the old tree).
+    // systemd-run puts the script in its own transient unit that outlives us;
+    // verified to survive on this machine. Fall back to a detached spawn for
+    // non-systemd sessions.
     const { spawn } = await import("child_process");
-    const child = spawn("/bin/sh", [script], { detached: true, stdio: "ignore" });
-    child.on("error", (err) => {
-      console.error("[folio] updater spawn failed:", err.message);
-    });
-    child.unref();
+    let handedOff = false;
+    try {
+      const { execFileSync } = await import("child_process");
+      execFileSync("systemd-run", [
+        "--user", "--quiet", "--collect",
+        `--description=Folio updater handoff`,
+        "/bin/sh", script,
+      ], { timeout: 10_000, stdio: "ignore" });
+      handedOff = true;
+    } catch {
+      try {
+        const child = spawn("/bin/sh", [script], { detached: true, stdio: "ignore" });
+        child.on("error", (err) => {
+          console.error("[folio] updater spawn failed:", err.message);
+        });
+        child.unref();
+        handedOff = true;
+      } catch (e) {
+        void e;
+      }
+    }
+    if (!handedOff) {
+      try { fs.appendFileSync(updateLog, new Date().toISOString() + " FAIL: could not start swap script\n"); } catch { /* ignore */ }
+      return { error: "Could not start the installer. Your current version is unchanged." };
+    }
     // Quit so the swap script can replace the app directory.
     setTimeout(() => app.quit(), 300);
     return { ok: true, version: rel.version };