Fix NameError: subprocess not defined in _spawn_transcribe (issue #6)

After v0.12.6/v0.12.7's HLS download fix, the next code path users hit was
_spawn_transcribe → subprocess.Popen, which crashed with NameError because
the function uses the bare name `subprocess` while the rest of the module
imports it locally as `import subprocess as _sp`.

Reproduces immediately when the user has --transcribe enabled during
download. Fix is a one-line module-level `import subprocess`.

User report (issue #6):
  Saved → /Volumes/.../EE4802IE4213Week1.mp4
  [transcribe] Starting background transcription: EE4802IE4213Week1.mp4
  [error] Unexpected error: name 'subprocess' is not defined

Author: nodeeeeee

downloader.py

@@ -24,6 +24,7 @@
 import os
 import random
 import re
+import subprocess
 import sys
 import time
 from datetime import datetime, timezone

electron/package.json

@@ -1,6 +1,6 @@
 {
   "name": "auto-note",
-  "version": "0.12.7",
+  "version": "0.12.8",
   "description": "AutoNote — lecture notes generator from Canvas recordings",
   "homepage": "https://github.com/nodeeeeee/Auto-Note",
   "author": {