Lay out the section nav's list items as flex rows again

Plan step 36, navigation review. The rewrite kept `min-w-0` from the
view's `<li class="flex min-w-0">` but dropped `flex`, so tabs.css's
`li > [data-md-tab] { flex: 1 1 0% }` no longer applied inside the
section nav. The header also called body-large "the tab bar's own"
type, which is title-small.

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

tests/Feature/Components/TabsTest.php

@@ -156,7 +156,10 @@ it('draws section navigation as secondary tabs and a picker', function () {
     expect($css->declarations('[data-md-section-nav-picker]', ['@media (width >= 600px)']))
         ->toBe(['display' => 'none'])
         ->and($css->declarations('[data-md-section-nav] > nav', ['@media (width < 600px)']))
-        ->toBe(['display' => 'none']);
+        ->toBe(['display' => 'none'])
+        // Each link's list item is a flex row, so tabs.css's `li > [data-md-tab]` shares its width.
+        ->and($css->declarations('[data-md-section-nav] [data-md-tabs-bar] > li'))
+        ->toBe(['display' => 'flex', 'min-inline-size' => '0']);
 });
 
 it('marks the section whose url is the request\'s, and scrolls many sections', function () {