feat(auth): password file management

Add muxplex/auth.py with password file management functions:
- get_password_path(): returns ~/.config/muxplex/password
- load_password(): reads password file, returns None if not found
- generate_and_save_password(): generates random password, writes with 0600 permissions

Add muxplex/tests/test_auth.py with 5 tests covering all functions.

Co-authored-by: Amplifier <amplifier@invalid>

muxplex/tests/test_auth.py

@@ -0,0 +1,63 @@
+"""Tests for muxplex/auth.py — authentication module."""
+
+import stat
+from pathlib import Path
+
+
+# ---------------------------------------------------------------------------
+# Password file management
+# ---------------------------------------------------------------------------
+
+
+def test_get_password_path_returns_expected_path(monkeypatch, tmp_path):
+    """get_password_path() returns ~/.config/muxplex/password."""
+    monkeypatch.setattr(Path, "home", staticmethod(lambda: tmp_path))
+    from muxplex.auth import get_password_path
+
+    assert get_password_path() == tmp_path / ".config" / "muxplex" / "password"
+
+
+def test_load_password_returns_none_when_no_file(monkeypatch, tmp_path):
+    """load_password() returns None when password file does not exist."""
+    monkeypatch.setattr(Path, "home", staticmethod(lambda: tmp_path))
+    from muxplex.auth import load_password
+
+    assert load_password() is None
+
+
+def test_load_password_reads_existing_file(monkeypatch, tmp_path):
+    """load_password() reads and strips the password file contents."""
+    monkeypatch.setattr(Path, "home", staticmethod(lambda: tmp_path))
+    pw_path = tmp_path / ".config" / "muxplex" / "password"
+    pw_path.parent.mkdir(parents=True, exist_ok=True)
+    pw_path.write_text("my-secret-password\n")
+    pw_path.chmod(0o600)
+
+    from muxplex.auth import load_password
+
+    assert load_password() == "my-secret-password"
+
+
+def test_generate_and_save_password_creates_file(monkeypatch, tmp_path):
+    """generate_and_save_password() creates the file and returns a non-empty string."""
+    monkeypatch.setattr(Path, "home", staticmethod(lambda: tmp_path))
+    from muxplex.auth import generate_and_save_password
+
+    pw = generate_and_save_password()
+    assert isinstance(pw, str)
+    assert len(pw) > 10
+
+    pw_path = tmp_path / ".config" / "muxplex" / "password"
+    assert pw_path.exists()
+    assert pw_path.read_text().strip() == pw
+
+
+def test_generate_and_save_password_sets_0600_permissions(monkeypatch, tmp_path):
+    """generate_and_save_password() sets the file to mode 0600."""
+    monkeypatch.setattr(Path, "home", staticmethod(lambda: tmp_path))
+    from muxplex.auth import generate_and_save_password
+
+    generate_and_save_password()
+    pw_path = tmp_path / ".config" / "muxplex" / "password"
+    mode = stat.S_IMODE(pw_path.stat().st_mode)
+    assert mode == 0o600