Remove PyQt5.Qwt dependency from the GUI client

PyQt5.Qwt isn't pip-installable; it only existed via the apt package
python3-pyqt5.qwt, so pip-installing client/requirements.txt's plain
PyPI PyQt5 always crashed with "No module named 'PyQt5.Qwt'".

Qwt was only used for 3 QwtSlider widgets (Slider_temp_soll,
Slider_pwr_soll, Slider_speed_soll). Swap them for stock QSlider in
brewpi.ui, regenerate brewpi_win.py via pyuic5, and update
brewpi_gui.py's setLowerBound/setUpperBound calls to QSlider's
integer-only setMinimum/setMaximum. The GUI now runs from a plain
`pip install -r client/requirements.txt` venv with no system PyQt5
packages required.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

@@ -80,7 +80,7 @@ commands back on (e.g. `{"TempCtrl": {"Soll": {"Temp": 65}}}`).
 - Python 3.8+
 - Server (`brewpi/requirements.txt`): `numpy`, `scipy`, `websockets`, `dpath`,
   and `pyserial`/`spidev` if using real hardware backends.
-- Client (`client/requirements.txt`): `PyQt5` (and `PyQt5.Qwt` for plotting).
+- Client (`client/requirements.txt`): `PyQt5`.
 
 Install with: