fix: add python-multipart dependency, fix launchd service name

python-multipart is required by FastAPI for form POST handling (login).
Was missing from pyproject.toml — crash on macOS first run.

launchd plist now uses the muxplex entry point script directly instead
of python -m muxplex, so macOS shows 'muxplex' in Activity Monitor,
launchctl list, and login items instead of 'python3'.

pyproject.toml

@@ -16,6 +16,7 @@ dependencies = [
     "websockets>=11.0",
     "python-pam>=1.8.4",
     "itsdangerous>=2.1.0",
+    "python-multipart>=0.0.9",
 ]
 
 [project.optional-dependencies]