build(deps): use the published dark-light release instead of its git repository

[pamburus]

The dark-light dependency was tracked from its git repository, which forced a pinned output hash in the Nix package. That pin is keyed by version, so it went stale as soon as the dependency's version moved — which is what breaks nix flake check on #695:

error: A hash was specified for dark-light-2.0.0, but there is no corresponding git dependency.

Version 3.0.0 is published on crates.io and carries the fixes the git revision was tracking, so the git source is no longer needed.

• Depend on dark-light 3.0.0 from crates.io
• Drop its now-unnecessary output hash from the Nix package

Resolving the registry version also pulls routine bumps of its transitive dependencies (zbus, zvariant, winreg) into the lockfile.

The detection API is unchanged between 2.0.0 and 3.0.0, so no code changes were needed.

Verification

• cargo clippy --locked --all-targets --all-features clean
• cargo test --locked --workspace passes
• cargo run --locked works
• nix flake check on this PR

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.58%. Comparing base (7e9c1ed) to head (ffb3944).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #696   +/-   ##
=======================================
  Coverage   80.58%   80.58%           
=======================================
  Files          31       31           
  Lines        4218     4218           
=======================================
  Hits         3399     3399           
  Misses        819      819           

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