Capture · Roots: roots drop w_full; hosts give the width

145 component roots drop w_full, the early returns included. Three
field rows keep it, Input, InputGroup and TagInput: stretched without
a width, taffy 0.9 laid the field's flex_1 share at no width in the
template picker at 280px, and the placeholder drew nothing. Container
keeps it, since mx_auto needs a width to fill.

Hosts that relied on a root's width now give it: MessageList boxes
each row at the list's width, the splash gives its bar the column's,
and test stages that mount a component as the window's root sit in a
size_full box. Tests hold a narrow row across the list and the splash
bar across its column; each goes red without its fix.

The gallery gains --narrow <px>, which lays each page in a padded
card at that width and shoots it without its script. The page
header wraps: the title takes the room and the theme switcher drops
below, where a long summary used to push it past the edge. A file
panel step waits for the panel to become key instead of a fixed
pause, which lost a race in a capture.

src/navigation/editor_tabs.rs

@@ -345,7 +345,6 @@ impl RenderOnce for EditorTabs {
             .id(self.id)
             .flex()
             .items_center()
-            .w_full()
             .bg(colors.sunken)
             .border_b_1()
             .border_color(colors.border)

src/navigation/goto.rs

@@ -83,7 +83,6 @@ impl RenderOnce for GoToLine {
             .flex()
             .flex_col()
             .gap_1()
-            .w_full()
             .capture_action(move |_: &Enter, window, cx| {
                 cx.stop_propagation();
                 match target(state.read(cx).text(), lines) {

src/navigation/steps.rs

@@ -333,7 +333,6 @@ impl RenderOnce for Wizard {
             .flex()
             .flex_col()
             .gap_6()
-            .w_full()
             .when(!self.headless, |wizard| {
                 wizard.child(
                     Steps::new((id.clone(), "steps"), self.steps, current).on_select(