feat(idle-grace-overlay): use fade to black for smooth transition with noctalia:dpms-off

[mattvilim]

Summary

I changed the idle grace color to black instead of shell surface because the transition is smooth in conjunction with idle dpms-off behavior (and won't change with wallpaper theme changes).

Motivation

Aesthetics are subjective, but I think this works as a better default. I considered adding a config field to let user choose but decided against it to avoid config bloat. Close if you disagree. 😃

Type of Change

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

Related Issue

N/A

Testing

Tested with:

[idle]
pre_action_fade_seconds = 4.0

[idle.behavior.screen-off]
enabled = true
timeout = 10
command = "noctalia:dpms-off"
resume_command = "noctalia:dpms-on"

Manual Coverage

• Tested on Niri
• Tested on Hyprland
• Tested on Sway
• Tested on another compositor:
• Tested with different bar positions and density settings
• Tested at different interface scaling values
• Tested with multiple monitors

Screenshots / Videos

N/A

Checklist

• This PR is ready for review, or it is marked as Draft.
• I read and followed the relevant guidance in CONTRIBUTING.md.
• I ran just format with clang-format v22+ installed, or this PR has no code changes.
• I ran the relevant build or test commands, or explained why they were not run.
• I self-reviewed the changes.
• I checked for new warnings or errors.
• I will update end-user documentation after merge, or this PR does not change user-facing configuration or behavior.
• I added or updated assets/translations/en.json, or this PR adds no new user-facing strings.
• I did not edit non-English translation files unless this PR is explicitly for translation tooling, an import/export sync, or a maintainer-requested locale change.
• I used the existing canonical names for config keys, IPC names, paths, and identifiers.

Additional Notes