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/views/MemoryView.vue

@@ -215,6 +215,10 @@ const displayUser = computed(() => (data.value?.user || '').replace(/§/g, '\n\n
   gap: 16px;
   flex: 1;
   min-height: 0;
+
+  @media (max-width: $breakpoint-mobile) {
+    flex-direction: column;
+  }
 }
 
 .memory-section {