feat: make when to auto-spawn a new stack configurable.

[cablehead]

It's current hard coded to 1 hour of inactivity: https://github.com/cablehead/stacks/blob/main/src-tauri/src/state.rs#L68

Something that annoys me is if you come back after a long break, and pop up stacks to create a new note, that'll go into the old stack, and make it current again.

We've had a suggestion to also add the option to start a new stack on laptop resume (when you open your laptop after closing it).

Document the behavior at the same time.