Sorry, but why do you think that (§616 BGB) should be able to be voided by an employment contract?
The company should be able to determine that your productivity isn’t good enough by the work you produce. Not micromanaging the keystrokes per minute.
If your work is really so unimportant that slacking off for 4+ hours a day isn’t noticed, they should be making you redundant. Not forcing spyware on every innocent employee
You too, it’s always nice to have productive polite discussion