feat: SEO complet + analytics Umami + og:image § logo

SEO :
- composable useSeoPage() : og:*, Twitter Cards, canonical sur toutes les pages (15 pages)
- app.vue : JSON-LD Organization + Book, og:image global og-default.png
- og-default.png 1200×630 : logo § calligraphique + texte (Pillow)
- nuxt.config.ts : @nuxtjs/sitemap avec 26 URLs statiques

Analytics Umami :
- useTracking() : helpers typés audio/pdf/player/scroll/cta
- useScrollTracking() : scroll depth 25/50/75/100% + liens externes auto
- useAudioPlayer : trackAudioPlay/Progress/Complete
- BookPdfReader : trackPdfOpen/Close avec durée
- BookPlayer : trackPlayerOpen/Chapter/Mode
- docker-compose : variables NUXT_PUBLIC_UMAMI_* passées au container

Images :
- Couv-Economie-du-don.jpg ajoutée dans public/images/
- bookplayer.config.yml + home.yml : références mises à jour

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

app/composables/useScrollTracking.ts

@@ -0,0 +1,55 @@
+/**
+ * Tracks scroll depth milestones (25 / 50 / 75 / 100 %) on each page.
+ * Also intercepts external link clicks and tracks them.
+ *
+ * Usage: call once in default.vue layout.
+ */
+export function useScrollTracking() {
+  const route = useRoute()
+  const { trackScrollDepth, trackExternalLink } = useTracking()
+
+  // ── Scroll depth ────────────────────────────────────────────────────────
+  const fired = new Set<number>()
+
+  function onScroll() {
+    const el = document.documentElement
+    const scrolled = el.scrollTop + el.clientHeight
+    const total = el.scrollHeight
+    if (total <= el.clientHeight) return
+    const pct = (scrolled / total) * 100
+
+    for (const milestone of [25, 50, 75, 100] as const) {
+      if (pct >= milestone && !fired.has(milestone)) {
+        fired.add(milestone)
+        trackScrollDepth(route.path, milestone)
+      }
+    }
+  }
+
+  // Reset fired milestones on route change
+  watch(() => route.path, () => fired.clear())
+
+  // ── External link tracking ───────────────────────────────────────────────
+  function onLinkClick(e: MouseEvent) {
+    const target = (e.target as HTMLElement).closest('a')
+    if (!target) return
+    const href = target.getAttribute('href') || ''
+    if (!href.startsWith('http') && !href.startsWith('//')) return
+    try {
+      const url = new URL(href)
+      if (url.hostname === 'librodrome.org') return
+      trackExternalLink(href, target.textContent?.trim() || '')
+    }
+    catch { /* invalid URL, ignore */ }
+  }
+
+  onMounted(() => {
+    window.addEventListener('scroll', onScroll, { passive: true })
+    document.addEventListener('click', onLinkClick)
+  })
+
+  onUnmounted(() => {
+    window.removeEventListener('scroll', onScroll)
+    document.removeEventListener('click', onLinkClick)
+  })
+}