fix: annotate float literals so the build is clean on Rust 1.97

[ayozetr]

cargo clippy --all-targets --locked -- -D warnings, the exact command in both CI jobs, fails on whatever dtolnay/rust-toolchain@stable currently resolves to (1.97.1) with 32 instances of:

falling back to `f32` as the trait bound `f32: From<f64>` is not satisfied

They are calls like Stroke::new(1.0, arc_border()), where the parameter is generic and an unsuffixed float literal no longer resolves the way it used to. rustc still accepts it but announces a future hard error, and -D warnings promotes it to a build failure today — so the job goes red on every PR regardless of what the PR touches.

I ran into this on the Linux job. The Windows one runs the identical command, and 27 of the 32 sit in theme.rs and widgets.rs, neither of which has any cfg(windows), so I would expect the same there — worth confirming, since I have no Windows machine to check on.

Mechanical change (1.0 → 1.0_f32) produced by cargo fix, which is where all 32 suggestions came from. Verified clean under 1.95 and 1.97 on Linux.

Sending this one on its own and first, since anything else lands red without it. Context in #8.

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