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/forms/catalogs.rs

@@ -138,13 +138,9 @@ impl RenderOnce for FontPicker {
         if let Some(on_change) = self.on_change {
             combobox = combobox.on_change(move |family, window, cx| on_change(family, window, cx));
         }
-        div()
-            .flex()
-            .flex_col()
-            .gap_2()
-            .w_full()
-            .child(combobox)
-            .when_some(self.selected, |picker, family| {
+        div().flex().flex_col().gap_2().child(combobox).when_some(
+            self.selected,
+            |picker, family| {
                 picker.child(
                     div()
                         .font_family(family)
@@ -152,7 +148,8 @@ impl RenderOnce for FontPicker {
                         .text_color(theme.colors.fg_muted)
                         .child("The quick brown fox jumps over the lazy dog"),
                 )
-            })
+            },
+        )
     }
 }