setMode() does not change refresh rate on Windows

[rodiaua]

Description

On Windows, calling Monitor.setMode() with a DisplayMode that has a different
refresh rate changes nothing. ChangeDisplaySettingsEx returns DISP_CHANGE_SUCCESSFUL (0),
but the monitor keeps its previous frequency.

Steps to reproduce

import pymonctl as pmc

mon = pmc.getPrimary()
print("current:", mon.mode)
print("available:", mon.allModes)

# pick a mode with the same resolution but a different frequency
target = next(m for m in mon.allModes
              if (m.width, m.height) == (mon.mode.width, mon.mode.height)
              and m.frequency != mon.mode.frequency)
mon.setMode(target)
print("after:", mon.mode)   # frequency is unchanged

Expected: the refresh rate changes to target.frequency.
Actual: the resolution is applied (if different), but the refresh rate stays the same.

Environment

• OS: Windows 11 Pro 25H2 / 26200.9457
• Python: 3.14.5
• PyMonCtl: 0.92
• pywin32: 312

Cause

In src/pymonctl/_pymonctl_win.py, setMode() sets devmode.DisplayFrequency, but
devmode.Fields doesn't include DM_DISPLAYFREQUENCY. Windows only reads the DEVMODE
members whose flags are set in Fields, so the frequency is ignored.

Proposed fix

-            devmode.Fields = win32con.DM_PELSWIDTH | win32con.DM_PELSHEIGHT
+            devmode.Fields = win32con.DM_PELSWIDTH | win32con.DM_PELSHEIGHT | win32con.DM_DISPLAYFREQUENCY

I've tested this fix locally and will open a PR.