Fix: run npm via shell in start_app.py so it works on Windows

[noramxiao]

Problem

On Windows, uv run start-app fails with FileNotFoundError during frontend setup. start_app.py runs the npm commands as:

subprocess.run(cmd.split(), cwd=frontend_dir, capture_output=True, text=True)

npm on Windows is a .cmd shim that CreateProcess can't execute directly, so passing a split argv (["npm", "install"]) raises FileNotFoundError. Reported in #126.

Change

subprocess.run(cmd, cwd=frontend_dir, capture_output=True, text=True, shell=True)

Passing the command string with shell=True resolves the executable through the platform shell on Windows (cmd.exe), macOS, and Linux (/bin/sh -c), so the same code works everywhere. This matches the fix suggested by the reporter.

Testing

Linux regression check

Ran the exact post-change invocation against the real e2e-chatbot-app-next frontend on Linux (node v24, npm 11.9):

subprocess.run("npm install", cwd=frontend_dir, capture_output=True, text=True, shell=True)   # rc=0
subprocess.run("npm run build", cwd=frontend_dir, capture_output=True, text=True, shell=True)  # rc=0, build complete

Both succeed, so shell=True with the string command is non-regressive on Linux. macOS uses the same /bin/sh -c path, so it's covered by the same mechanism.

Windows (the platform the fix targets) still needs a manual uv run start-app check — the .cmd shim / CreateProcess failure can't be reproduced off Windows.

Fixes #126