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/App.vue

@@ -1,5 +1,5 @@
 <script setup lang="ts">
-import { onMounted, onUnmounted, computed, ref } from 'vue'
+import { onMounted, onUnmounted, computed, ref, watch } from 'vue'
 import { useRoute, useRouter } from 'vue-router'
 import { NConfigProvider, NMessageProvider, NDialogProvider, NNotificationProvider } from 'naive-ui'
 import { themeOverrides } from '@/styles/theme'
@@ -14,6 +14,11 @@ const ready = ref(false)
 
 const isLoginPage = computed(() => route.name === 'login')
 
+// Close mobile sidebar on route change
+watch(() => route.path, () => {
+  appStore.closeSidebar()
+})
+
 // Wait for router to resolve before rendering layout
 router.isReady().then(() => {
   ready.value = true
@@ -39,6 +44,10 @@ useKeyboard()
       <NDialogProvider>
         <NNotificationProvider>
           <div v-if="ready" class="app-layout" :class="{ 'no-sidebar': isLoginPage }">
+            <button v-if="!isLoginPage" class="hamburger-btn" @click="appStore.toggleSidebar">
+              <svg width="20" height="20" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round"><line x1="3" y1="6" x2="21" y2="6"/><line x1="3" y1="12" x2="21" y2="12"/><line x1="3" y1="18" x2="21" y2="18"/></svg>
+            </button>
+            <div v-if="!isLoginPage && appStore.sidebarOpen" class="mobile-backdrop" @click="appStore.closeSidebar" />
             <AppSidebar v-if="!isLoginPage" />
             <main class="app-main">
               <router-view />