fix(egui): suffix Stroke width literals to satisfy float_literal_f32_fallback

Rust's new float_literal_f32_fallback lint (rust-lang/rust#154024) denies
bare untyped literals passed where `impl Into<f32>` is expected, since
`f32: From<f64>` doesn't hold. CI floats to the latest stable toolchain
via dtolnay/rust-toolchain@stable, so this started failing `cargo xtask
verify` there before it showed up on any dev machine still on an older
stable. Suffix every affected Stroke::new width literal with _f32.