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/models/ProviderFormModal.vue

@@ -150,7 +150,7 @@ function handleClose() {
     v-model:show="showModal"
     preset="card"
     :title="t('models.addProvider')"
-    :style="{ width: '520px' }"
+    :style="{ width: 'min(520px, calc(100vw - 32px))' }"
     :mask-closable="!loading"
     @after-leave="emit('close')"
   >

src/components/models/ProvidersPanel.vue

@@ -48,7 +48,7 @@ const modelsStore = useModelsStore()
 
 .providers-grid {
   display: grid;
-  grid-template-columns: repeat(auto-fill, minmax(420px, 1fr));
+  grid-template-columns: repeat(auto-fill, minmax(min(100%, 420px), 1fr));
   gap: 14px;
 }
 </style>