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/navigation-bar.blade.php

@@ -24,12 +24,12 @@
      down and springs back on a scroll up, never before the first screenful has gone by, and never
      while a snackbar, a bottom sheet or a drawer is on screen — those are anchored to the bar's
      edge and would slide with it. Focus reaching the bar brings it back, which is as close as the
-     web gets to M3's "never hide it while a screen reader is active". Inside `<x-app-shell>`
+     web gets to M3's "never hide it while a screen reader is active". Inside `<x-scaffold>`
      `--material-bottom-bar` follows the bar down and up, so a `fab` button, the snackbar and the
      page's bottom padding keep their distance from it rather than from where it was.
 
      It does not position itself: wrap it in the element that pins it (`fixed inset-x-0 bottom-0`)
-     and hides it where a rail takes over. `<x-app-shell>` does both, and lifts the snackbar and a
+     and hides it where a rail takes over. `<x-scaffold>` does both, and lifts the snackbar and a
      `fab` button above it through `--material-bottom-bar`.
 
      `label` names the landmark ("Main" by default); `tall` picks the 80px container;