Add optional=true to niri configs

[LuckShiba]

Description

Closes #1534. Adds optional=true to the includes in DMS niri-flake module and embedded/niri.kdl config. Now that niri 26.04 is mainstreamed, we can add this to avoid niri config parsing failing for still not generated files, such as in #1534.

Also updated flake.lock as a routinary action.

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that changes existing behavior)
• Refactor / internal cleanup
• Documentation
• Other

Related issues

Screenshots / video

Checklist

• My code follows the conventions in CONTRIBUTING.md
• I have tested my changes locally
• New user-facing strings are wrapped in I18n.tr() with translator context, reusing existing terms where possible
• Go changes: ran make fmt, added/updated tests, make test passes, and go mod tidy is clean
• QML changes: ran make lint-qml with no new warnings
• I have opened a corresponding pull request in dlx-docs to document any new behaviors: https://github.com/AvengeMedia/DankLinux-Docs