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/forms/structure.rs

@@ -55,7 +55,6 @@ impl RenderOnce for Form {
             .flex()
             .flex_col()
             .gap_8()
-            .w_full()
             .when_some(self.on_submit, |form, submit| {
                 form.capture_action(move |_: &Submit, window, cx| {
                     cx.stop_propagation();
@@ -102,7 +101,6 @@ impl RenderOnce for FormSection {
             .flex()
             .flex_col()
             .gap_4()
-            .w_full()
             .child(
                 div()
                     .flex()
@@ -304,7 +302,6 @@ impl RenderOnce for InlineForm {
             .flex_wrap()
             .items_end()
             .gap_3()
-            .w_full()
             .children(self.children)
     }
 }