fix: follow setting when quick scheduling next week (#3373)

[miqh]

Description

Hello there,

I tried scouring history for context around why the quick action for scheduling a task forces it to always be the following Monday, but it's still not clear to me why. The following commit appears to be the original appearance:

https://github.com/johannesjo/super-productivity/commit/88220015d225249b4c84e7384b616c05177be73d

The proposed changes here should ensure that the FIRST_DAY_OF_WEEK setting is respected.

Though one might argue that rather than abide by the setting, the action should instead be today's date plus seven days.

Anyway, I'm happy to revise as necessary. I was going to pull out the rather similar logic for establishing the scheduled date into a utility function, but thought I'd solicit feedback first.

Issues Resolved

#3373

Check List

• [ ] New functionality includes testing.
• [ ] New functionality has been documented in the README if applicable.