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/chat/MessageItem.vue

@@ -467,4 +467,18 @@ const formattedToolResult = computed(() => {
     transform: scale(1);
   }
 }
+
+@media (max-width: $breakpoint-mobile) {
+  .msg-user .msg-body {
+    max-width: 90%;
+  }
+
+  .msg-assistant .msg-body {
+    max-width: 92%;
+  }
+
+  .msg-system .msg-body {
+    max-width: 92%;
+  }
+}
 </style>