system_info.py cannot find the current power_now value for a power supply

### Error output:
```text
auto-cpufreq --stats

Using settings defined in /etc/auto-cpufreq.conf file

----------------------------------- Warning -----------------------------------

Detected you are running a TLP service!
This daemon might interfere with auto-cpufreq which can lead to unexpected results.
We strongly encourage you to remove TLP unless you really know what you are doing.
press Enter to continue or Ctrl + c to exit...
Traceback (most recent call last):
File "/usr/bin/auto-cpufreq", line 8, in
sys.exit(main())
~~~~^^
File "/usr/lib/python3.13/site-packages/click/core.py", line 1161, in __call__
return self.main(*args, **kwargs)
~~~~~~~~~^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.13/site-packages/click/core.py", line 1082, in main
rv = self.invoke(ctx)
File "/usr/lib/python3.13/site-packages/click/core.py", line 1443, in invoke
return ctx.invoke(self.callback, **ctx.params)
~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.13/site-packages/click/core.py", line 788, in invoke
return __callback(*args, **kwargs)
File "/usr/lib/python3.13/site-packages/auto_cpufreq/bin/auto_cpufreq.py", line 236, in main
monitor.run()
~~~~~~~~~~~^^
File "/usr/lib/python3.13/site-packages/auto_cpufreq/modules/system_monitor.py", line 297, in run
self.loop.run()
~~~~~~~~~~~~~^^
File "/usr/lib/python3.13/site-packages/urwid/event_loop/main_loop.py", line 337, in run
self._run()
~~~~~~~~~^^
File "/usr/lib/python3.13/site-packages/urwid/event_loop/main_loop.py", line 439, in _run
self.event_loop.run()
~~~~~~~~~~~~~~~~~~~^^
File "/usr/lib/python3.13/site-packages/urwid/event_loop/select_loop.py", line 182, in run
self._loop()
~~~~~~~~~~^^
File "/usr/lib/python3.13/site-packages/urwid/event_loop/select_loop.py", line 224, in _loop
alarm_callback()
~~~~~~~~~~~~~~^^
File "/usr/lib/python3.13/site-packages/urwid/event_loop/main_loop.py", line 218, in cb
callback(self, user_data)
~~~~~~~~^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.13/site-packages/auto_cpufreq/modules/system_monitor.py", line 98, in update
report: SystemReport = system_info.generate_system_report()
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^
File "/usr/lib/python3.13/site-packages/auto_cpufreq/modules/system_info.py", line 333, in generate_system_report
battery_info=self.battery_info(),
~~~~~~~~~~~~~~~~~^^
File "/usr/lib/python3.13/site-packages/auto_cpufreq/modules/system_info.py", line 263, in battery_info
energy_rate: str | None = read_file(
~~~~~~~~~^
f"{POWER_SUPPLY_DIR}{supply}/power_now"
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
)
^
File "/usr/lib/python3.13/site-packages/auto_cpufreq/modules/system_info.py", line 221, in read_file
return f.read().strip()
~~~~~~^^
OSError: [Errno 19] No such device
```
---

### System information:

```
Using settings defined in /etc/auto-cpufreq.conf file

-------------------------------------------------------------------------------

Linux distro: Arch Linux rolling n/a
Linux kernel: 6.14.2-arch1-1
Processor: 12th Gen Intel(R) Core(TM) i7-1255U
Cores: 12
Architecture: x86_64
Driver: intel_pstate

------------------------------ Current CPU stats ------------------------------

CPU max frequency: 1700 MHz
CPU min frequency: 400 MHz

Core Usage Temperature Frequency
CPU0 2.0% 35 °C 1316 MHz
CPU1 0.0% 35 °C 400 MHz
CPU2 1.0% 37 °C 518 MHz
CPU3 0.0% 37 °C 400 MHz
CPU4 2.0% 36 °C 1450 MHz
CPU5 1.0% 36 °C 400 MHz
CPU6 1.0% 36 °C 1701 MHz
CPU7 3.0% 36 °C 1700 MHz
CPU8 2.0% 33 °C 400 MHz
CPU9 0.0% 33 °C 1520 MHz
CPU10 0.0% 33 °C 400 MHz
CPU11 0.0% 33 °C 400 MHz

CPU fan speed: 0 RPM

auto-cpufreq version: Version : 2.6.0-1

Python: 3.13.2
psutil package: 7.0.0
platform package: 1.0.8
click package: 8.1.8
distro package: 1.9.0

Computer type: Convertible
Battery is: charging

auto-cpufreq system resource consumption:
cpu usage: 0.0 %
memory use: 0.3 %

Total CPU usage: 1.2 %
Total system load: 2.11
Average temp. of all cores: 35.00 °C

Currently using: performance governor
Currently turbo boost is: on

-------------------------------------------------------------------------------
```
### What's the issue?

When testing the new release of `auto-cpufreq` to release to the AUR, I have noted that when running `auto-cpufreq --stats`, it does not work as intended, since it crashes, which is very clearly not intended.

Within the file `auto_cpufreq/modules/system_info.py`, it tries to find the statistic located at `{POWER_SUPPLY_DIR}{supply}/power_now`, which expands to `/sys/class/power_supply/[DEVICE]/power_now`, which doesn't exist for neither my mouse nor my charger. However, the code seems to handle the charger, but not the mouse. When I unplug both devices, it still errors out, which is quite counter-intuitive, even after restarting `auto-cpufreq`. My belief is that the code does not correctly handle the mains device, and thinks it is a battery, even when it is not. Note that the mains device remains there, even when unplugged.

I believe this stems from the refactoring from the commit https://github.com/AdnanHodzic/auto-cpufreq/commit/081dbda79c3ea9adf9dc2436d3bdbbae993c80ab, since this command worked fine before this commit from my testing.

---