fix(nix): declare notification bus readiness

[domenkozar]

Description

The NixOS and Home Manager modules declared DMS as started before its QuickShell process acquired org.freedesktop.Notifications. Consumers ordered after dms.service could therefore initialize too early and permanently fall back to non-system notifications.

Configure both module variants as Type=dbus with BusName=org.freedesktop.Notifications. Update the service-start test double to acquire that name and assert the readiness contract in both module tests.

This lets dependent user services reliably use After=dms.service.

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

None.

Screenshots / video

Not applicable; this changes service startup ordering.

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

Validation:

• nix flake check --no-build --show-trace
• nix build .#nixosTests.x86_64-linux.nixos-service-start-module
• repository pre-commit hooks