Run clippy and fmt

[LeWimbes]

Summary

Split out of #69 – this PR fixes clippy warnings and formats the entire workspace.

As the final step from #69, I ran cargo clippy and cargo fmt again.

Many changes suggested by clippy were related to uninlined format arguments (see the clippy lint.
If you’d prefer to silence that lint instead of inlining, we could add

[workspace.lints.clippy]
uninlined_format_args = "allow"

clippy still reports two warnings that you might want to have a look at yourself:

1. Undocumented unsafe function: rustecal-service/src/types.rs:42
2. main function inside a doc test: rustecal/src/lib.rs:16

Checklist

• I have tested this change locally
• I have documented any public APIs or CLI changes
• I have added appropriate examples or comments
• The code builds and passes all checks (cargo check, cargo test)
• I have updated the changelog if applicable

[rex-schilasky]

I will take care for the warnings that are left in a separate PR.

[LeWimbes]

I'm not sure why the clippy workflow shows different warnings than clippy does on my device. I thought it might be due to a version mismatch, but both the workflow and the devcontainer use version 1.88.0.