feat: add center focused on overflow, rework center_focused setting

[Yocraft-2000]

Summary

Added on_overflow option for the center_focused setting.

center_focused = "never"

accepted values are "never", "always", or "on_overflow".

The on_overflow option will center if it doesn't fit on screen together with the previously focused column.

This is a breaking change, previous config is a bool, now it's a string.

Motivation

Type of Change

• Bug fix
• New feature
• Breaking change
• Refactoring
• Build / packaging
• Documentation

Related Issue

Closes #100.

Testing

just format
just test
just lint

Manual Coverage

• Tested in a nested Umbriel session
• Tested in a native Umbriel session
• Tested with multiple monitors
• Tested with a scaled output
• Tested with native Wayland applications
• Tested with X11 applications through xwayland-satellite
• Tested with the scrolling layout
• Tested with the dwindle layout

Screenshots / Videos

Checklist

• This PR is ready for review, or it is marked as Draft.
• This change fits SCOPE.md, or its scope was agreed in an issue or on Discord first.
• I read and followed the relevant guidance in CONTRIBUTING.md.
• I ran just format, or this PR has no C++ changes.
• I ran the relevant build, test, lint, or verification commands, or explained why they were not run.
• I functionally verified compositor behavior where automated checks are insufficient.
• I self-reviewed the changes.
• I checked for new warnings or errors.
• I updated docs/ and examples/config.toml, or this PR does not change user-facing configuration or behavior.
• I used canonical names for config keys, IPC actions, paths, and identifiers.

Additional Notes

The tests needs to be updated.