Display greeter on xwayland: primary display only

[jerezereh]

Addresses issue #435.

Uses the xwayland: primary configuration to determine primary monitor to display login subsurface on, falling back to leftmost-display (as oriented by hardware defaults, will not pick up user configuration of monitor setup), and destroys subsurfaces on non-primary displays. Tested and working with lock screen, suspend, and restart greeter paths.

Known issues:

• The detect-and-destroy strategy makes a race condition in the greeter more visible, where the first monitor to connect receives focus, rather than the primary. This issue already exists (Password input field is out of focus - possible regression #286), but is currently hidden by duplicating the login subsurface across all displays. I have investigated this issue and determined it likely requires changes outside of this repo.
• When login box is unfocused, a fallback inputs uncaptured character text into the password field, but keys with no text output fail (backspace, enter, etc.). Through testing I found that focus is being taken away after being established in greeter.rs, likely through some lower-level GUI mechanism. Related issues include multiple displays connected. login box not selected. #110 and Tab should focus password input #71.

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Disclaimer: Being new to Rust, I have used AI agent Codex for reference and review. If anything looks incorrect or out of place, please leave feedback or guidance.