feat(discover): content discovery panel with For You, tags, playlists, collections (M13.1-M13.8)

- For You feed: frequency map from favorites, sorted by most-favorited type
- Content tagging: user tags on any item, tag cloud, filter by tag
- Smart playlists: recently played, most played, by genre, by decade
- Similar content: background AI call for 3 suggestions, cached 7 days
- Recently viewed history: last 50 items with time-ago display
- Content collections: user-curated mixed-type lists with mosaic grid
- Trending: most-referenced items across 30 days with badge
- Share to Nostr: compose preview, sign via NIP-07, broadcast to relays

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

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

@@ -45,7 +45,7 @@
             : isDark
               ? 'text-white/40 hover:text-white/70 hover:bg-white/5'
               : 'text-gray-500 hover:text-gray-800 hover:bg-black/5'"
-          @click="tab === 'favorites' ? (activeTab = tab) : setActiveTab(tab)"
+          @click="tab === 'favorites' || tab === 'discover' ? (activeTab = tab) : setActiveTab(tab)"
         >
           {{ tabLabel(tab) }}
         </button>
@@ -168,6 +168,9 @@
           <FavoritesGrid
             v-else-if="activeTab === 'favorites'"
           />
+          <DiscoverPanel
+            v-else-if="activeTab === 'discover'"
+          />
         </ErrorBoundary>
       </div>
     </aside>
@@ -194,6 +197,7 @@ import MagazineGrid from './MagazineGrid.vue'
 import ProjectGrid from './ProjectGrid.vue'
 import NostrGrid from './NostrGrid.vue'
 import FavoritesGrid from './FavoritesGrid.vue'
+import DiscoverPanel from './DiscoverPanel.vue'
 import ErrorBoundary from '@/components/ui/ErrorBoundary.vue'
 import { useFavoritesStore } from '@/stores/favorites'
 
@@ -259,6 +263,9 @@ const displayTabs = computed(() => {
   if (favoritesStore.items.length > 0 && !tabs.includes('favorites')) {
     tabs.push('favorites')
   }
+  if (!tabs.includes('discover')) {
+    tabs.push('discover')
+  }
   return tabs
 })
 
@@ -281,6 +288,7 @@ const TAB_LABELS: Record<ContentTab, string> = {
   'design-system': 'Design',
   nostr: 'Nostr',
   favorites: 'Favorites',
+  discover: 'Discover',
 }
 
 function tabLabel(tab: ContentTab): string {