Document desktop compatibility

[Staphylococcus]

Summary

• Replace the README compatibility prose with a compact feature table.
• Show support across GNOME, non-GNOME Wayland with swayidle, and other Linux sessions.
• Keep backend-specific caveats in the notes column and add lg-buddy detect-backend to day-to-day commands.

Impact

This makes it clearer that LG Buddy is not GNOME-only while still being explicit about which idle-monitoring features require GNOME or swayidle.

Validation

• git diff --cached --check before commit
• git diff --check after README edits

[Copilot]

Pull request overview

This PR updates the README to clarify LG Buddy’s desktop-session support by replacing narrative compatibility text with a compact feature matrix, making it explicit that GNOME is optional while still documenting where GNOME- or swayidle-specific behavior applies.

Changes:

• Replaced the README compatibility prose with a “Desktop Compatibility” feature table covering GNOME, non-GNOME Wayland + swayidle, and other Linux sessions.
• Updated backend prerequisites and added optional swayidle install hints for common distros.
• Added lg-buddy detect-backend to the “Day to Day” command list.

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