Rust 1.85.0 + Rust 2024 Edition (#68)

- Add `rust-toolchain.toml`
- Refactor `flake` to consider `rust-toolchain.toml`, especially Rust version
- `just`: Add run-sound command
- Ignore sound files
- Format

src/duration.rs

@@ -1,6 +1,6 @@
 use color_eyre::{
-    eyre::{ensure, eyre},
     Report,
+    eyre::{ensure, eyre},
 };
 use std::fmt;
 use std::time::Duration;