fix: (re)read pipewire nodes if empty

[nohn]

pw-dump was only executed on pw-mixer start. so pipewire nodes had be active pw-mixer start to be recognized.

so

1. start pw-mixer
2. start google chrome

resulted in the fader for google chrome not working

this pr fixes this by re-reading the config if idX.isEmpty()

[nohn]

Not nice about this PR: It tries to re-read the config on each MIDI message, so if a MIDI fader is moved from 0 to 127 while the id being empty, config is re-read 127 times

[nohn]

As a side note: Source comments say, such an update happens every 100 MIDI messages, but I couldn't find an implementation of this. Instead of trying to fix that, I implemented this PR which IMHO is a more efficient approach.

[Katacc]

I removed the automatic pw-dump at some release and added a keybind for CYCLE key to trigger manual pw-dump to avoid high latency on changing volumes. Ill look at this implementation when I got time

[nohn]

To clarify: This is not about deliberate changes to the config, for that the manual reload via cycle button is a perfect solution. This is about the pipewire node.name not being present in certain (not so uncommon) conditions, so fetching the ID fails:

Variant 1:

1. A software configured in pw-mixer (for example scrcpy) hasn't been started yet: $ pw-dump | grep node.name | grep -i scrcpy =>
2. As a consequence, id for the configured fader is empty

Variant 2:

1. A software (I guess it's not all software behaving like this, but for example Google Chrome or Microsoft Edge do) is not currently playing audio when pw-mixer started:

user@host:~ $ pw-dump | grep node.name | grep -i chrome
user@host:~ $ pw-dump | grep application.name | grep -i chrome
        "application.name": "Google Chrome input",

vs

user@host:~ $ $ pw-dump | grep node.name | grep -i chrome
        "node.name": "Google Chrome",
user@host:~ $ pw-dump | grep application.name | grep -i chrome
        "application.name": "Google Chrome input",
        "application.name": "Google Chrome",
        "application.name": "Google Chrome",

2. As a consequence, id for the configured fader is empty

Without my PR it's like this:
Screencast from 2025-12-22 16-35-55.webm
And yes, this can be fixed by pressing the "cycle" button.

With my PR it's like this:
Screencast from 2025-12-22 16-37-52_opt.webm