Rename the app shell to the scaffold and give it a FAB slot

Plan step 35: <x-app-shell> becomes <x-scaffold>, M3's and Compose's name
for the structure of bars, rails and panes, with no alias; every package
view, the showcase, the tests, the skills, README and UPGRADE follow. Its
markup and Tailwind classes stay for the navigation rewrite. The new fab
slot places the FAB as Compose's Scaffold does - bottom-end, 16px from the
edges below medium and 24px from it, clear of the navigation bar and of a
snackbar on screen (resources/css/layout/scaffold.css) - and the content
region tells a pane inside that the margin is already drawn.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Qwx5USif3wFFmxtHg5U1g9

resources/views/components/toolbar.blade.php

@@ -25,7 +25,7 @@
      A docked toolbar and a navigation bar occupy the same region of the screen and M3 says never to
      show both at once: the bar belongs on a primary page, the toolbar on a secondary or contextual
      one. Either way a toolbar placed at `bottom` clears `--material-bottom-bar`, so it is never
-     buried under `<x-app-shell>`'s bar.
+     buried under `<x-scaffold>`'s bar.
 
      Put `<x-button icon="…" tooltip="…" />` controls in the slot (`:selected` for toggles). It is a
      `role="toolbar"`: the arrow keys move between its controls (resources/js/toolbar.js,