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/ContentPanel.vue

@@ -140,6 +140,9 @@
         <ProjectGrid
           v-else-if="activeTab === 'code'"
         />
+        <NostrGrid
+          v-else-if="activeTab === 'nostr'"
+        />
       </div>
     </aside>
   </Transition>
@@ -163,6 +166,7 @@ import NewsGrid from './NewsGrid.vue'
 import ArticleDetail from './ArticleDetail.vue'
 import MagazineGrid from './MagazineGrid.vue'
 import ProjectGrid from './ProjectGrid.vue'
+import NostrGrid from './NostrGrid.vue'
 
 const { isDark } = useTheme()
 const {
@@ -186,6 +190,7 @@ const {
   selectedSong,
   selectedPodcast,
   selectedArticle,
+  selectedDesignSystemItem,
   setActiveTab,
   openFilmDetail,
   closeFilmDetail,
@@ -202,7 +207,7 @@ const {
 } = useContentPanel()
 
 const hasDetailOpen = computed(() =>
-  !!(selectedFilm.value || selectedBook.value || selectedTVSeries.value || selectedSong.value || selectedPodcast.value || selectedArticle.value)
+  !!(selectedFilm.value || selectedBook.value || selectedTVSeries.value || selectedSong.value || selectedPodcast.value || selectedArticle.value || selectedDesignSystemItem.value)
 )
 
 const windowWidth = ref(window.innerWidth)
@@ -224,6 +229,8 @@ const TAB_LABELS: Record<ContentTab, string> = {
   websites: 'Web',
   magazine: 'Brief',
   code: 'Code',
+  'design-system': 'Design',
+  nostr: 'Nostr',
 }
 
 function tabLabel(tab: ContentTab): string {