Add logging functionality for installer process, capturing stdout, stderr, and error details to a timestamped log file in the downloads directory.

public/appupdate.js

@@ -176,6 +176,35 @@ const getMacAppPath = (app) => {
 
 const quoteShellArg = (value) => `'${String(value).replaceAll("'", "'\\''")}'`
 
+const writeInstallerLog = async (app, { installerPath, stdout, stderr, error }) => {
+  const downloadsDir = app.getPath('downloads')
+  const timestamp = new Date().toISOString().replaceAll(':', '-').replaceAll('.', '-')
+  const logPath = path.join(
+    downloadsDir,
+    `farmcontrol-update-install-${timestamp}.log`
+  )
+
+  const sections = [
+    `Timestamp: ${new Date().toISOString()}`,
+    `Installer: ${installerPath}`,
+    `Status: ${error ? 'failed' : 'success'}`,
+    '',
+    '--- stdout ---',
+    stdout || '(empty)',
+    '',
+    '--- stderr ---',
+    stderr || '(empty)'
+  ]
+
+  if (error) {
+    sections.push('', '--- error ---', error.stack || error.message || String(error))
+  }
+
+  await fs.writeFile(logPath, `${sections.join('\n')}\n`, 'utf8')
+  console.log('[app-update] installer log written to:', logPath)
+  return logPath
+}
+
 const getInstallErrorMessage = (error, output = '') => {
   const combined = `${output}\n${error?.message || ''}`.trim()
 
@@ -244,6 +273,15 @@ const launchMacInstaller = (app, installerPath, webContents) => {
       if (stdout) console.log('[app-update] installer stdout:', stdout)
       if (stderr) console.error('[app-update] installer stderr:', stderr)
 
+      void writeInstallerLog(app, {
+        installerPath,
+        stdout,
+        stderr,
+        error
+      }).catch((logError) => {
+        console.error('[app-update] failed to write installer log:', logError)
+      })
+
       if (error) {
         console.error('[app-update] installer error:', error)
         const message = getInstallErrorMessage(error, output)