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/chat/media.rs

@@ -72,7 +72,6 @@ impl RenderOnce for ImageMessage {
         let widest = theme.prose_width().to_pixels(window.rem_size());
         let open = self.on_open;
         div()
-            .w_full()
             .flex()
             .flex_col()
             .gap_1()
@@ -235,7 +234,6 @@ impl RenderOnce for FileMessage {
         let theme = cx.theme();
         let colors = theme.colors.clone();
         div()
-            .w_full()
             .max_w(theme.prose_width())
             .flex()
             .items_center()
@@ -339,7 +337,6 @@ impl RenderOnce for LinkPreviewCard {
         let open = self.on_open;
         div()
             .id(self.id.clone())
-            .w_full()
             .max_w(theme.container_width(ContainerSize::Sm))
             .flex()
             .gap_3()