Draft: Recentre as part of stage measure thing

Closes #499

Julian, review this before I'm ready and I'll assign it to you

This MR contains the following

• Currently, a separate recentre thing

Before merge:

• Combine into the stage measure thing
• Replace lateral offset with percent of FOV
• Tests

Merge checklist:

• All new/changed functions have up to date typehints and docstrings
• Any UI updates have a before and after screenshot in the MR description
• Unit testing of new functionality has similar or better coverage than similar existing code
• This code has been tested on a microscope
  • Webapp has been recompiled, with aesthetic changes checked in light and dark mode (if changed)
  • With updated settings JSON (if changed)
  • SmartScan and live stitching work