fix(grip): popup sizes to widest label + full-row highlight

The previous single-widget refactor left the column at
`Length::Shrink` and the buttons at `Length::Fill`, which iced
resolves to a zero-width column — the popup degenerated into a
1-pixel vertical line. Set the column to a fixed pixel width
derived from the longest item's label (and restore `width(Fill)`
on the buttons) so the selection highlight covers the full row
instead of just the text glyphs.

Also silences a `dead_code` warning on `GripMenuAction` variants
that the Phase 2 / 3 wiring will consume next.

src/scene/object.rs

@@ -111,6 +111,7 @@ pub struct GripMenuItem {
 /// default no-op-equivalent — picking it just starts the regular
 /// stretch drag, identical to clicking the grip with no popup open.
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
+#[allow(dead_code)]
 pub enum GripMenuAction {
     Stretch,
     Lengthen,