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/composables/useContentPanel.ts

@@ -50,6 +50,7 @@ const contentType = ref<'film' | 'song' | 'podcast'>('film')
 const activeTab = ref<ContentTab>('film')
 const availableTabs = ref<ContentTab[]>([])
 const selectedDesignSystemItem = ref<DesignSystemItem | null>(null)
+const longFormArticle = ref<{ content: string; title?: string } | null>(null)
 
 export interface DesignSystemItem {
   id: string
@@ -271,6 +272,7 @@ export function useContentPanel() {
     selectedWebsite.value = null
     selectedMagazineSection.value = null
     selectedDesignSystemItem.value = null
+    longFormArticle.value = null
   }
 
   function openFilmDetail(film: Film) { clearAllSelections(); selectedFilm.value = film }
@@ -291,6 +293,9 @@ export function useContentPanel() {
   function openWebsiteDetail(website: WebSearchResult) { clearAllSelections(); selectedWebsite.value = website; panelOpen.value = true }
   function closeWebsiteDetail() { selectedWebsite.value = null }
 
+  function openLongFormArticle(content: string, title?: string) { clearAllSelections(); longFormArticle.value = { content, title }; panelOpen.value = true }
+  function closeLongFormArticle() { longFormArticle.value = null }
+
   function openMagazineSectionDetail(section: MagazineSection, index: number) {
     clearAllSelections()
     selectedMagazineSection.value = section
@@ -435,6 +440,9 @@ export function useContentPanel() {
     selectedDesignSystemItem,
     openDesignSystemItem,
     closeDesignSystemItem,
+    longFormArticle,
+    openLongFormArticle,
+    closeLongFormArticle,
     enterDesignSystemMode,
     closePanel,
     showAllFilms,