fix: back up existing systemd unit before overwriting on update (#24350) (#24937)

When `openclaw update` regenerates the systemd service file, any user
customizations to ExecStart (e.g. proxychains4 wrapper) are silently
lost. Now the existing unit file is copied to `.bak` before writing
the new one, so users can restore their customizations.

The backup path is printed in the install output so users are aware.

Co-authored-by: echoVic <AkiraVic@outlook.com>

src/daemon/systemd.ts

@@ -193,6 +193,18 @@ export async function installSystemdService({
 
   const unitPath = resolveSystemdUnitPath(env);
   await fs.mkdir(path.dirname(unitPath), { recursive: true });
+
+  // Preserve user customizations: back up existing unit file before overwriting.
+  let backedUp = false;
+  try {
+    await fs.access(unitPath);
+    const backupPath = `${unitPath}.bak`;
+    await fs.copyFile(unitPath, backupPath);
+    backedUp = true;
+  } catch {
+    // File does not exist yet — nothing to back up.
+  }
+
   const serviceDescription = resolveGatewayServiceDescription({ env, environment, description });
   const unit = buildSystemdUnit({
     description: serviceDescription,
@@ -227,6 +239,14 @@ export async function installSystemdService({
         label: "Installed systemd service",
         value: unitPath,
       },
+      ...(backedUp
+        ? [
+            {
+              label: "Previous unit backed up to",
+              value: `${unitPath}.bak`,
+            },
+          ]
+        : []),
     ],
     { leadingBlankLine: true },
   );