fix(mpris): keep the service importable without dbus-next

The stub dbus_property() returned a bare function, so the read/write
properties (@Volume.setter and friends) raised AttributeError at import
time. That made any host without dbus-next -- macOS, Windows, minimal
containers -- crash before the TUI could start, because app.py imports
this module unconditionally.

Replace the stub with a descriptor that supports .getter()/.setter(), and
give the service a short label for the status bar.

somafm_tui/mpris.py

@@ -44,9 +44,39 @@ except Exception:  # pragma: no cover
         def emit_properties_changed(self, *args: Any, **kwargs: Any) -> None:
             return None
 
+    class _StubProperty:  # type: ignore[no-redef]
+        """Stand-in for ``dbus_next.service.dbus_property`` on D-Bus-less hosts.
+
+        It has to behave like a descriptor that also supports ``.setter()``,
+        because the interfaces below define read/write properties with the
+        usual decorator idiom.  Importing this module must never fail just
+        because there is no session bus (macOS, Windows, minimal containers).
+        """
+
+        def __init__(self, fget: Any = None) -> None:
+            self.fget = fget
+            self.fset: Any = None
+
+        def getter(self, fget: Any) -> "_StubProperty":
+            self.fget = fget
+            return self
+
+        def setter(self, fset: Any) -> "_StubProperty":
+            self.fset = fset
+            return self
+
+        def __get__(self, obj: Any, objtype: Any = None) -> Any:
+            if obj is None or self.fget is None:
+                return self
+            return self.fget(obj)
+
+        def __set__(self, obj: Any, value: Any) -> None:
+            if self.fset is not None:
+                self.fset(obj, value)
+
     def dbus_property(*args: Any, **kwargs: Any):  # type: ignore[no-redef]
-        def decorator(function):
-            return function
+        def decorator(function: Any) -> _StubProperty:
+            return _StubProperty(function)
 
         return decorator
 
@@ -254,6 +284,9 @@ class _PlayerInterface(ServiceInterface):
 
 
 class MprisService:
+    #: Status-bar name for this backend (see :mod:`somafm_tui.mediasession`).
+    label = "mpris"
+
     def __init__(
         self, actions: "queue.Queue[str]", enabled: bool = True
     ) -> None: