feat(ui): build MegaMenu and rebuild Sidebar on Drawer

MegaMenu replaces a scaffold with a trigger and a wide panel of grouped link
columns. One level deep on purpose: a mega menu exists to show breadth flat so
everything is one click away, and nesting inside the panel buries content
behind hover-within-hover. Nav is the component for cascading submenus. The
panel is anchored so the runtime clamp keeps it inside the viewport.

Sidebar now composes Drawer for its off-canvas presentation instead of a
hand-rolled backdrop, inheriting the focus trap and scroll lock from one
place. Single items, labelled groups and branches nested to three levels, with
vertical roving focus.

Sidebar classes move to the BEM naming the rest of the library uses, which is
a breaking change; nesting via children still works, since that is what
shipped in 0.8.5.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

examples/component-showcase/app/pages/index.wrn

@@ -17,11 +17,11 @@ page ComponentShowcase {
       <section class="home-product-grid"><article><span class="icon-[lucide--component] size-7"></span><strong>108 components</strong><p>Accessible primitives for every product surface.</p><a href="/base">Browse components →</a></article><article><span class="icon-[lucide--layout-template] size-7"></span><strong>Ready-made blocks</strong><p>Composable sections assembled from WRNexus UI.</p><a href="/block-library">Explore blocks →</a></article><article><span class="icon-[lucide--panels-top-left] size-7"></span><strong>Page templates</strong><p>Complete responsive pages ready to customize.</p><a href="/templates">View templates →</a></article></section>
       <section class="showcase-stats">
         <div class="showcase-stat"><strong>108</strong><span>Unique components</span></div>
-        <div class="showcase-stat"><strong>421</strong><span>Live configurations</span></div>
+        <div class="showcase-stat"><strong>420</strong><span>Live configurations</span></div>
         <div class="showcase-stat"><strong>0</strong><span>Duplicate implementations</span></div>
         <div class="showcase-stat"><strong>11</strong><span>Focused categories</span></div>
       </section>
-      <section class="home-categories"><div class="home-section-heading"><span>Explore the system</span><h2>Everything your product needs</h2></div><div class="home-category-grid"><a class="home-category home-category--1" href="/advanced-forms"><span class="home-category-index">01</span><div><strong>Advanced Forms</strong><p>8 components · 112 live demos</p></div><span class="icon-[lucide--arrow-up-right] size-5" aria-hidden="true"></span></a><a class="home-category home-category--2" href="/base"><span class="home-category-index">02</span><div><strong>Base</strong><p>27 components · 86 live demos</p></div><span class="icon-[lucide--arrow-up-right] size-5" aria-hidden="true"></span></a><a class="home-category home-category--3" href="/core"><span class="home-category-index">03</span><div><strong>Core</strong><p>5 components · 13 live demos</p></div><span class="icon-[lucide--arrow-up-right] size-5" aria-hidden="true"></span></a><a class="home-category home-category--4" href="/data"><span class="home-category-index">04</span><div><strong>Data</strong><p>3 components · 9 live demos</p></div><span class="icon-[lucide--arrow-up-right] size-5" aria-hidden="true"></span></a><a class="home-category home-category--1" href="/forms"><span class="home-category-index">05</span><div><strong>Forms</strong><p>12 components · 36 live demos</p></div><span class="icon-[lucide--arrow-up-right] size-5" aria-hidden="true"></span></a><a class="home-category home-category--2" href="/integrations"><span class="home-category-index">06</span><div><strong>Integrations</strong><p>11 components · 33 live demos</p></div><span class="icon-[lucide--arrow-up-right] size-5" aria-hidden="true"></span></a><a class="home-category home-category--3" href="/layout"><span class="home-category-index">07</span><div><strong>Layout</strong><p>15 components · 45 live demos</p></div><span class="icon-[lucide--arrow-up-right] size-5" aria-hidden="true"></span></a><a class="home-category home-category--4" href="/marketing"><span class="home-category-index">08</span><div><strong>Marketing</strong><p>10 components · 30 live demos</p></div><span class="icon-[lucide--arrow-up-right] size-5" aria-hidden="true"></span></a><a class="home-category home-category--1" href="/navigation"><span class="home-category-index">09</span><div><strong>Navigation</strong><p>10 components · 30 live demos</p></div><span class="icon-[lucide--arrow-up-right] size-5" aria-hidden="true"></span></a><a class="home-category home-category--2" href="/overlays"><span class="home-category-index">010</span><div><strong>Overlays</strong><p>6 components · 18 live demos</p></div><span class="icon-[lucide--arrow-up-right] size-5" aria-hidden="true"></span></a><a class="home-category home-category--3" href="/tables"><span class="home-category-index">011</span><div><strong>Tables</strong><p>1 components · 9 live demos</p></div><span class="icon-[lucide--arrow-up-right] size-5" aria-hidden="true"></span></a></div></section>
+      <section class="home-categories"><div class="home-section-heading"><span>Explore the system</span><h2>Everything your product needs</h2></div><div class="home-category-grid"><a class="home-category home-category--1" href="/advanced-forms"><span class="home-category-index">01</span><div><strong>Advanced Forms</strong><p>8 components · 112 live demos</p></div><span class="icon-[lucide--arrow-up-right] size-5" aria-hidden="true"></span></a><a class="home-category home-category--2" href="/base"><span class="home-category-index">02</span><div><strong>Base</strong><p>27 components · 86 live demos</p></div><span class="icon-[lucide--arrow-up-right] size-5" aria-hidden="true"></span></a><a class="home-category home-category--3" href="/core"><span class="home-category-index">03</span><div><strong>Core</strong><p>5 components · 13 live demos</p></div><span class="icon-[lucide--arrow-up-right] size-5" aria-hidden="true"></span></a><a class="home-category home-category--4" href="/data"><span class="home-category-index">04</span><div><strong>Data</strong><p>3 components · 9 live demos</p></div><span class="icon-[lucide--arrow-up-right] size-5" aria-hidden="true"></span></a><a class="home-category home-category--1" href="/forms"><span class="home-category-index">05</span><div><strong>Forms</strong><p>12 components · 36 live demos</p></div><span class="icon-[lucide--arrow-up-right] size-5" aria-hidden="true"></span></a><a class="home-category home-category--2" href="/integrations"><span class="home-category-index">06</span><div><strong>Integrations</strong><p>11 components · 33 live demos</p></div><span class="icon-[lucide--arrow-up-right] size-5" aria-hidden="true"></span></a><a class="home-category home-category--3" href="/layout"><span class="home-category-index">07</span><div><strong>Layout</strong><p>15 components · 45 live demos</p></div><span class="icon-[lucide--arrow-up-right] size-5" aria-hidden="true"></span></a><a class="home-category home-category--4" href="/marketing"><span class="home-category-index">08</span><div><strong>Marketing</strong><p>10 components · 30 live demos</p></div><span class="icon-[lucide--arrow-up-right] size-5" aria-hidden="true"></span></a><a class="home-category home-category--1" href="/navigation"><span class="home-category-index">09</span><div><strong>Navigation</strong><p>10 components · 29 live demos</p></div><span class="icon-[lucide--arrow-up-right] size-5" aria-hidden="true"></span></a><a class="home-category home-category--2" href="/overlays"><span class="home-category-index">010</span><div><strong>Overlays</strong><p>6 components · 18 live demos</p></div><span class="icon-[lucide--arrow-up-right] size-5" aria-hidden="true"></span></a><a class="home-category home-category--3" href="/tables"><span class="home-category-index">011</span><div><strong>Tables</strong><p>1 components · 9 live demos</p></div><span class="icon-[lucide--arrow-up-right] size-5" aria-hidden="true"></span></a></div></section>
     </div>
   }
 }