Migrate ssr example to use FeathersRadio and FeathersNumberInput (Phase 5 Item 6)

[kfc35]

Objective

• Part of UI Migration Plan for Bevy Examples #24112

Solution

• Reuse the helpers/radio.rs helper that creates FeathersRadio for the toggles, improving as necessary so that both the pcss and ssr examples look good.
• Use FeathersNumberInput for the number inputs.
• bsn!-ify the ui portion of the example only.

Testing

• cargo run --example ssr --features="bluenoise_texture bevy_feathers” Everything toggles as expected. The number inputs work like they do in the feathers_gallery example, where you can drag to slide or insert numbers directly. The only thing may be that the words become hard to read when switching to the metallic base.
• cargo run --example pcss --features="bevy_feathers experimental_pbr_pcss” still looks OK with the text color changes.

One thing I noticed while testing this time is that the FeathersNumberInput stops accepting keyboard input after I command+shift+4 or command+shift+5 to screenshot or record something on my screen. The slider still works but the keyboard input doesn’t. I spun this out as #25041