list: Implement `InteractiveElement` for `ListItem` to support drag and drop (#2639)

## Description

Implement `InteractiveElement` and `StatefulInteractiveElement` for
`ListItem` by delegating to the inner `Stateful<Div>`, the same pattern
used by `Button`, `Checkbox`, `Radio`, and `Tab`.

- All GPUI native interaction APIs (`on_drag`, `on_drop`, `drag_over`,
`can_drop`, `on_hover`, `tooltip`, ...) become directly available on
`ListItem`, so rows can act as drag sources and drop targets inside
`Tree` or any custom list.
- Inherent `on_click` / `on_mouse_down` / `on_mouse_enter` keep their
precedence and disabled/separator gating, so existing consumers are
unaffected.
- Add a "Draggable" toggle to the List story that demonstrates real
reordering on drop.
- Document drag-and-drop usage in the List docs (en and zh-CN).

This supersedes #2553. Exposing the interactive traits gives the full
native API surface with unchanged semantics, instead of forwarding
`on_drag` / `on_drop` / `on_drag_hover` one method at a time
(`on_drag_hover` there is actually plain `on_hover`, which also fires
outside of drags).

## How to Test

- `cargo test -p gpui-component --lib list_item` — 3 passed
- `cargo clippy -- --deny warnings`
- `cargo run -- List`, check "Draggable", then drag a row onto another
row: a preview follows the cursor, an indicator line on the target row's
top or bottom edge shows the insertion position, and dropping reorders
the list. Click selection still works.