Feathers: `pane_header()` and `pane_body()` border inconsistency

`pane_header()`'s Node is bordered while `pane_body()`'s isn't which produces a visual inconsistency, with a jarring 1 pixel offset:

Adding the same border on the body's left, bottom and right sides feels more pleasing overall, as illustrated by this comparison on `feathers_gallery`:

Open question is how to deal with headerless panes, which should add a top border. As a matter of fact they should also fix their top corners radii too, to mach the bottom ones.

Is this too naive ?
```rust
pub fn pane_body(header_less: bool) -> impl Scene {}
```

Can the body detect the presence of a sibling header automatically ?

Locate the Feathers implementations of pane_header() and pane_body(), then compare their border and corner-radius behavior in the feathers_gallery example. Check how headerless panes are represented or rendered before deciding whether the body needs explicit context. Done means the panes have consistent borders and corner radii, including the intended headerless case.

Written by the indexing model from the issue text.