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.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/shell/dialogs.rs

@@ -93,7 +93,6 @@ impl RenderOnce for AboutDialog {
             .px_8()
             .pt_4()
             .pb_8()
-            .w_full()
             .child(mark(&self.name, self.icon, cx))
             .child(div().pt_3().child(Title::new(self.name.clone())))
             .child(CopyableText::new(
@@ -234,7 +233,7 @@ impl RenderOnce for SplashScreen {
                     .flex_col()
                     .items_center()
                     .gap_2()
-                    .child(track)
+                    .child(div().w_full().child(track))
                     .child(
                         div()
                             .text_size(theme.text_size(TextSize::Xs))