style(design): FINDING-003 — fix heading hierarchy (h3 → h2)

Sidebar section headings were h3 while page titles are h1, skipping h2.
Change to h2 for proper heading hierarchy (h1 → h2, no skip).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/shared/components/layout/SlideMenu.vue

@@ -15,7 +15,7 @@
         :key="section.title"
         class="menu-section"
       >
-        <h3 class="menu-title">{{ section.title }}</h3>
+        <h2 class="menu-title">{{ section.title }}</h2>
         <ul class="menu-list">
           <li
             v-for="item in getFilteredItems(section.items)"