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/mail/list.rs

@@ -290,7 +290,6 @@ impl RenderOnce for MailList {
         if self.mails.is_empty() {
             return div()
                 .debug_selector(|| "mail-none".into())
-                .w_full()
                 .px_3()
                 .py_4()
                 .text_size(theme.text_size(TextSize::Sm))
@@ -320,7 +319,6 @@ impl RenderOnce for MailList {
         let (id, on_select) = (self.id.clone(), self.on_select);
         div()
             .debug_selector(|| "mail-list".into())
-            .w_full()
             .child(
                 list.selected(self.selected)
                     .on_change(move |keys, window, cx| {