Fix Widget changed layer_id panic when undocking tabs (debug builds)

[mcthesw]

Problem

The tab body Ui was constructed with an ID that didn't include the surface:

let id = self.id.with(tab_viewer.id(tab));

When a tab is moved to a floating window surface, both the main dock (Background layer) and the window (Middle layer) can render a tab body with the same ID in the same frame. egui's debug assertion then fires because it sees the same widget ID appearing in two different layers.

Fix

Scope the tab body ID by surface:

fn tab_body_id(dock_area_id: Id, path: NodePath, tab_id: Id) -> Id {
    dock_area_id.with((path.surface, "surface")).with(tab_id)
}

This preserves the original property that tab IDs remain stable when a tab is moved between nodes on the same surface, while ensuring tabs on different surfaces get distinct IDs.

closes #317