Key the views on M3's window size classes, not Tailwind's

Plan step 15. Every `sm:`/`md:`/`lg:`/`xl:` in the components, the showcase
(sections, layout, shell) and the pagination and error views becomes the M3
window size class the audits' breakpoint maps prescribe: the full-screen
dialog, the FAB, the snackbar's width, `radio inline`, the section picker and
the page numbers move from 640px to `medium` (600px, M3's compact boundary,
N-07 / C-07 / IN-28); `button responsive` moves from 1024px to `expanded`
(840px); `section-nav`'s grid-to-row switch from 1280px to `large` (1200px,
N-07); and `drawer pane` from 1280px to `expanded`, where M3 shows two panes
for list-detail (C-07). The pane's default width is now 360dp, M3's fixed
supporting pane at expanded.

Showcase example grids take two columns from `medium` and three or more from
`expanded`; the showcase's own theme picker swaps at `expanded`, the nearest
class boundary to the 768px it used (no M3 rule applies to it).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Qwx5USif3wFFmxtHg5U1g9

resources/views/showcase/sections/containment.blade.php

@@ -1,7 +1,7 @@
 @php
     $examples = [
         'Cards' => <<<'BLADE'
-            <div class="grid w-full gap-4 md:grid-cols-3">
+            <div class="grid w-full gap-4 expanded:grid-cols-3">
                 <x-card title="Filled" subtitle="surface-container-highest">
                     The default card.
                     <x-slot:actions><x-button label="Open" /></x-slot:actions>
@@ -27,7 +27,7 @@
             </x-card>
             BLADE,
         'Lists' => <<<'BLADE'
-            <div class="grid w-full gap-6 md:grid-cols-2">
+            <div class="grid w-full gap-6 medium:grid-cols-2">
                 <x-list dividers label="Files">
                     <x-list-item title="holiday-photos.zip" description="248 MB" icon="folder_zip" trailing="3 days" />
                     <x-list-item title="contract.pdf" overline="Password protected" description="1.2 MB · downloaded twice" icon="picture_as_pdf" trailing="1 hour" />