rust-toolchain file is misnamed, so the toolchain is not pinned

Describe the bug
The file at the repository root is named toml .rust-toolchain.toml, with a leading toml in the filename. rustup only reads rust-toolchain.toml or rust-toolchain, so it is ignored. Nothing is pinned locally or in CI; both use whatever stable is installed.

The README claimed twice that the toolchain was pinned. PR #178 corrected the README text but not the file.

Expected behavior
Either the file is renamed so the pin takes effect, or it is deleted so it stops implying a pin that does not exist.

Additional context
Renaming is not a safe one-line fix. The file pins channel = "1.80.0", and relay depends on chrono 0.4.42 and an unpinned iced, which may not build on 1.80. Whoever takes this should verify a full build on the pinned version before renaming, or raise the pin to something tested.

Found while writing the setup documentation in #138 / #178.

Start at the repository-root file named toml .rust-toolchain.toml and inspect its channel = "1.80.0" pin, then review the README claims and the chrono 0.4.42 and unpinned iced dependencies. Run a full build with the pinned version before deciding whether to rename or delete the file, or change the pin. Done means the toolchain behavior matches the README and the selected configuration is verified to build.