fix(terminal): stand the docked composer as far in from the bottom as from the sides

Docked under the grid, the box sits inside the pane's padding, and only
the side inset took it off: the box stood 20px up from the pane's bottom
and 16px in from its sides, where it covers an agent's input 16px from
all three.

src/terminal/composer.rs

@@ -1563,8 +1563,9 @@ impl TerminalView {
                 .flex_none()
                 .w_full()
                 .pt(px(8.))
+                // The pane's own padding is already between it and the edges.
                 .px(px(BOX_INSET - GRID_PAD_X))
-                .pb(px(BOX_INSET))
+                .pb(px(BOX_INSET - GRID_PAD_Y))
                 .child(frame)
                 .into_any_element(),
         };