feat(renderer): full article reader with TOC, font controls & print (M10.1)

- ArticleReader.vue: sticky TOC sidebar (desktop), mobile dropdown
- Auto-generated heading anchors with IntersectionObserver tracking
- Reading time estimate, font size controls (persisted localStorage)
- Print mode opens clean window with serif typography
- Long-form detection (>800 words + headings) shows "Read as article"
- Removed isDark conditionals from ArticleDetail (dark-only app)

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

packages/app/src/components/chat/ChatMessage.vue

@@ -277,6 +277,13 @@
         >
           View brief →
         </button>
+        <button
+          v-if="isLongForm"
+          class="text-[10px] text-accent/70 hover:text-accent transition-colors"
+          @click.stop="openLongFormArticle(message.content)"
+        >
+          Read as article →
+        </button>
       </div>
       </div>
     </div>
@@ -364,7 +371,7 @@ function submitEdit() {
   isEditing.value = false
   editContent.value = ''
 }
-const { getContextualInlineContent, stripContentTags, stripMarkdownLinks, updatePanelFromText, panelOpen, availableTabs, setActiveTab, openFilmDetail, openBookDetail, openTVSeriesDetail, openImageDetail, openPlaceDetail, openSongDetail, openPodcastDetail, openArticleDetail, openWebsiteDetail, closeFilmDetail, closeBookDetail, closeTVSeriesDetail, closeImageDetail, closePlaceDetail, closeSongDetail, closePodcastDetail } = useContentPanel()
+const { getContextualInlineContent, stripContentTags, stripMarkdownLinks, updatePanelFromText, panelOpen, availableTabs, setActiveTab, openFilmDetail, openBookDetail, openTVSeriesDetail, openImageDetail, openPlaceDetail, openSongDetail, openPodcastDetail, openArticleDetail, openWebsiteDetail, openLongFormArticle, closeFilmDetail, closeBookDetail, closeTVSeriesDetail, closeImageDetail, closePlaceDetail, closeSongDetail, closePodcastDetail } = useContentPanel()
 const codeContext = useCodeContext()
 
 const isUser = computed(() => props.message.role === 'user')
@@ -459,6 +466,15 @@ const formattedTime = computed(() => {
   return d.toLocaleTimeString([], { hour: '2-digit', minute: '2-digit' })
 })
 
+// Long-form article detection (>800 words + has headings)
+const isLongForm = computed(() => {
+  if (isUser.value) return false
+  const text = props.message.content
+  const words = text.split(/\s+/).length
+  const hasHeadings = /^#{2,3}\s+.+$/m.test(text)
+  return words > 800 && hasHeadings
+})
+
 // Token estimate (~4 chars per token)
 const estimatedTokens = computed(() => Math.ceil(props.message.content.length / 4))
 const showTokenCount = computed(() => props.message.content.length > 20)