Fix font sizes not reacting on settings change (#26060)

Proper version of https://github.com/zed-industries/zed/pull/25425
When https://github.com/zed-industries/zed/pull/24857 returned font
updates on settings changes, settings values, not in-memory ones should
be compared.

This PR returns back the logic finally, and changes it to explicitly
track the settings values, not the in-memory ones.
Also adds the same tracking for UI font changes, which had never been
tracked before.

Release Notes:

- Fixed font sizes not reacting on settings change