feat: add mobile responsiveness support

- Hamburger menu + drawer sidebar for mobile navigation
- Auto-collapse chat session list on mobile
- Responsive grids, modals, forms, and settings
- Touch-friendly nav items (44px targets)
- Skills page sidebar toggle on mobile
- Memory sections stack vertically on mobile

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/components/layout/LanguageSwitch.vue

@@ -24,7 +24,7 @@ function handleChange(val: string) {
     :options="options"
     size="tiny"
     :consistent-menu-width="false"
-    style="width: 90px"
+    class="input-sm"
     @update:value="handleChange"
   />
 </template>