🐛 fix(av1-transfer): find sh dynamically via shutil.which

/bin/sh doesn't exist on this system, sh is at /usr/bin/sh.
Use shutil.which to locate the shell at runtime.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

av1-transfer/main.py

@@ -21,6 +21,7 @@ import ctypes
 import ctypes.util
 import queue
 import shlex
+import shutil
 import subprocess
 import sys
 from concurrent.futures import ThreadPoolExecutor, as_completed
@@ -133,9 +134,11 @@ def transcode_one_file(file_info, overall_bar, slot_bars, slot_queue, process_na
             # 用 bash 的 exec -a 重命名 ffmpeg 进程的 argv[0]
             # ffmpeg 通过 PATH 查找，避免 execve 直接调用 symlink 的兼容问题
             shell_args = " ".join(shlex.quote(str(a)) for a in ffmpeg_args)
+            sh = shutil.which("sh") or shutil.which("bash") or "/usr/bin/sh"
             proc = subprocess.Popen(
                 f"exec -a {shlex.quote(process_name)} ffmpeg {shell_args}",
                 shell=True,
+                executable=sh,
                 stdout=subprocess.PIPE,
                 stderr=subprocess.PIPE,
             )