Migrate GUI to Qt6

[martukas]

Description

This is blocked until new RpiOS distros begin supporting Qt6, or else we somehow deploy this GUI in a different way...

Closes #1329

Self-review checklist:

• Self-review: looked through the Files changed tab, browsed repository in branch
• Documentation updated - reflects changes in code, electrical or mechanical design
• Documentation and graphics follow the documentation style guide
• New content is linked, easily discoverable, does not require too many clicks
• Follows other relevant parts of contributor wiki
• PR has a descriptive name
• Tagged relevant reviewers

Software only:

• Follows our code style
• Commented code, particularly in hard-to-understand areas
• All tests pass - new and old, locally and on CI
• No new warnings or static check failures
• Tests that prove fix is effective or new feature works
• Manual tests are explained, with instructions for reproducing them