feat: add voice pitch shifting

F6/F7 adjust outgoing voice pitch in 1-semitone steps (-12 to +12).
PitchShifter integrates into the capture path at dequeue time so the
PortAudio callback is never blocked.  Status bar shows current pitch
when non-zero.  Config reload treats pitch as a safe change.

pyproject.toml

@@ -11,6 +11,7 @@ dependencies = [
     "textual>=1.0.0",
     "pymumble>=1.6",
     "sounddevice>=0.5.0",
+    "numpy>=1.24.0",
     "tomli>=2.0.0;python_version<'3.11'",
 ]