feat(app): add design system viewer, nostr feed, stop generation, and content refactor

- Design system browser with grid/detail views for tokens and components
- Nostr feed tab with note/article/zap filtering and relay status
- Stop generation button to abort AI streaming mid-response
- Paste & extract content without sending to AI
- Refactor useContentPanel into contentExtraction.ts and contentFiltering.ts
- Banner fallback composable for 3-stage image loading
- Wikipedia and Google Books as fallback image sources
- Loading skeletons with variant-specific shapes
- Mobile UX: auto-switch to content, back button, detail flow
- Project grid with breadcrumb nav and inline creation
- Filesystem Vite plugin for local project browsing
- Magazine text cleanup and song grid polish

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

packages/app/src/components/content/DetailView.vue

@@ -56,6 +56,11 @@
     v-else-if="isCodeMode && activeCodeFile"
     @back="closeCodeFile"
   />
+  <DesignSystemDetail
+    v-else-if="selectedDesignSystemItem"
+    :item="selectedDesignSystemItem"
+    @back="closeDesignSystemItem"
+  />
 </template>
 
 <script setup lang="ts">
@@ -71,6 +76,7 @@ import ArticleDetail from './ArticleDetail.vue'
 import WebsiteDetail from './WebsiteDetail.vue'
 import MagazineSectionDetail from './MagazineSectionDetail.vue'
 import CodeDetail from './CodeDetail.vue'
+import DesignSystemDetail from './DesignSystemDetail.vue'
 import { useCodeContext } from '@/composables/useCodeContext'
 
 const { isCodeMode, activeFile: activeCodeFile } = useCodeContext()
@@ -105,5 +111,7 @@ const {
   panelMagazineSections,
   closeMagazineSectionDetail,
   navigateMagazineSection,
+  selectedDesignSystemItem,
+  closeDesignSystemItem,
 } = useContentPanel()
 </script>