[feature] Default "Re-stock detection setting" in settings

**Version and OS**
0.49.1 on linux/docker

**Is your feature request related to a problem? Please describe.**
I'm always frustrated when I forgot to change this option and then I have to open all my watches to be sure that it's set

**Describe the solution you'd like**
Global Setting in Settings what option is default when create/save watch in mode "Re-stock & Price detection for single product pages"

Re-stock detection:
In Stock only (Out Of Stock -> In Stock only)
Any availability changes
Off, don't follow availability/restock

Now "In Stock only (Out Of Stock -> In Stock only)" is always selected by default, but I always prefer to set "Any availability changes". So it would be perfect to set it as default.

Start by locating the Settings implementation and the create/save-watch flow for “Re-stock & Price detection for single product pages.” Trace where the current “In Stock only” default is selected, then verify that a saved global preference controls the default for new watches while existing watches remain unaffected.

Written by the indexing model from the issue text.