Hide a disabled fab button on a compact window, as M3 removes a FAB

`<x-button fab>` is an extended FAB below `medium` and a filled button
from there, but a disabled one was still drawn as the FAB below
`medium`: the compact FAB's rule set no state of its own, so the button
stood greyed out and fixed over the content. M3 never disables a FAB:
"if its action is unavailable, remove the FAB entirely".

A `fab` given `disabled` (a `<button disabled>`, or a link's
`aria-disabled="true"`) now renders `data-md-unavailable`, and
button.css draws it `display: none` below `medium`, which also takes it
out of the accessibility tree and the Tab order; from `medium` it is
the disabled filled button it was. The mark comes from the prop, not
from `:disabled`, because a `spinner` puts `disabled` on its button
while the action runs, and that FAB is busy rather than unavailable:
hidden, it would vanish instead of showing its loading indicator. The
docblocks and the skill's button table say so.

A browser test renders an enabled fab with a slow spinner action, a
disabled one and a disabled link on a 393px window: the disabled two
are not rendered, the busy one stays on screen with its indicator, and
at 600px both disabled ones are the disabled button again; it fails
without the change in Chrome, Firefox and Safari (and in Chrome with
`:disabled` alone as the condition, on the busy FAB). A feature test
reads the mark and the rule.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

resources/views/components/button.blade.php

@@ -36,7 +36,9 @@
      pinned in the thumb zone on a compact window (below `medium`, 600px), a filled button from
      there — one element either way. It lifts clear of a bottom bar and of a snackbar on screen
      (`--material-bottom-bar`, `--material-snackbar-height`), because M3 puts a snackbar above a
-     FAB and never over one.
+     FAB and never over one. A `fab` that is `disabled` is not drawn below `medium`, since M3
+     removes a FAB whose action is unavailable rather than showing it disabled, and is the disabled
+     filled button from there; a spinner's own disabling while the action runs leaves it on screen.
 
      The view renders the props as `data-md-*` attributes and resources/css/components/button.css
      draws them, in the package's components layer: a caller's class or unlayered CSS outranks
@@ -124,6 +126,9 @@
         'data-md-width' => $iconOnly ? $width : null,
         'data-md-selected' => $selected === null ? null : ($selected ? 'true' : 'false'),
         'data-md-compact-fab' => $fab ? true : null,
+        // Read from the prop, not from `:disabled`: a spinner disables its button while the action
+        // runs, and that FAB is busy, not unavailable.
+        'data-md-unavailable' => $fab && $disabled ? true : null,
         'href' => $isLink ? $link : null,
         'target' => $isLink && $external ? '_blank' : null,
         'rel' => $isLink && $external ? 'noopener' : null,