feat : notion de livre — grouper des pages en série ordonnée

Ajoute un concept de "livre" (série de pages ordonnées) avec :
- BookManager : CRUD JSON dans data/books/<slug>.json
- Route /book/<slug> → page de sommaire (table des matières)
- Navigation chapitre ← → en bas de chaque article membre du livre
- Bandeau "Chapitre X/N — Nom du livre" en haut de l'article
- Admin → onglet Livres : créer, éditer, supprimer un livre, ajouter/ordonner les pages via textarea slug

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

public/assets/css/style.css

@@ -1807,3 +1807,171 @@ footer.mt-5 { margin-top: 0 !important; }
     color: var(--vl-muted);
     margin-top: 0.15rem;
 }
+
+/* ─── Livres ─────────────────────────────────────────────────────── */
+
+/* Bandeau dans un article appartenant à un livre */
+.book-article-banner {
+    border-radius: var(--vl-radius);
+    background: var(--vl-accent-soft);
+    border: 1px solid rgba(79,70,229,.18);
+    overflow: hidden;
+}
+.book-article-banner-link {
+    display: flex;
+    align-items: center;
+    gap: 0.6rem;
+    padding: 0.6rem 1rem;
+    text-decoration: none;
+    color: var(--vl-accent);
+    transition: background 0.15s;
+}
+.book-article-banner-link:hover {
+    background: rgba(79,70,229,.08);
+    color: var(--vl-accent-dark);
+}
+.book-article-banner-icon { font-size: 1.1rem; flex-shrink: 0; }
+.book-article-banner-text { flex: 1; font-size: 0.875rem; }
+.book-article-banner-cta  { font-size: 0.8rem; opacity: .75; white-space: nowrap; }
+
+/* Navigation précédent/suivant en bas d'article */
+.book-chapter-nav {
+    border-top: 1px solid var(--vl-border);
+    margin-top: 1.5rem;
+    padding-top: 1rem;
+}
+.book-chapter-nav-inner {
+    display: flex;
+    gap: 0.75rem;
+    align-items: stretch;
+}
+.book-nav-btn {
+    flex: 1;
+    display: flex;
+    flex-direction: column;
+    padding: 0.65rem 0.875rem;
+    background: var(--vl-surface);
+    border: 1px solid var(--vl-border);
+    border-radius: var(--vl-radius);
+    text-decoration: none;
+    color: var(--vl-text);
+    transition: border-color 0.15s, box-shadow 0.15s;
+    min-width: 0;
+}
+.book-nav-btn:hover {
+    border-color: var(--vl-accent);
+    box-shadow: var(--vl-shadow-sm);
+    color: var(--vl-text);
+}
+.book-nav-btn--disabled {
+    opacity: .45;
+    cursor: default;
+    pointer-events: none;
+}
+.book-nav-btn--next { text-align: right; }
+.book-nav-dir {
+    font-size: 0.72rem;
+    font-weight: 600;
+    text-transform: uppercase;
+    letter-spacing: .04em;
+    color: var(--vl-muted);
+    display: block;
+}
+.book-nav-title {
+    font-size: 0.875rem;
+    font-weight: 500;
+    display: block;
+    margin-top: 0.15rem;
+    overflow: hidden;
+    display: -webkit-box;
+    -webkit-line-clamp: 2;
+    -webkit-box-orient: vertical;
+}
+.book-nav-toc {
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    padding: 0.5rem 0.75rem;
+    border: 1px solid var(--vl-border);
+    border-radius: var(--vl-radius);
+    color: var(--vl-muted);
+    text-decoration: none;
+    font-size: 1.1rem;
+    transition: border-color 0.15s, color 0.15s;
+    flex-shrink: 0;
+}
+.book-nav-toc:hover {
+    border-color: var(--vl-accent);
+    color: var(--vl-accent);
+}
+
+/* Page sommaire d'un livre (/book/<slug>) */
+.book-page { max-width: 720px; margin: 0 auto; padding: 2rem 0; }
+.book-label {
+    font-size: 0.75rem;
+    font-weight: 700;
+    text-transform: uppercase;
+    letter-spacing: .08em;
+    color: var(--vl-accent);
+}
+.book-chapters {
+    list-style: none;
+    padding: 0;
+    margin: 0;
+    display: flex;
+    flex-direction: column;
+    gap: 0.5rem;
+}
+.book-chapter-link {
+    display: flex;
+    align-items: center;
+    gap: 0.875rem;
+    padding: 0.75rem 1rem;
+    background: var(--vl-surface);
+    border: 1px solid var(--vl-border);
+    border-radius: var(--vl-radius);
+    text-decoration: none;
+    color: var(--vl-text);
+    transition: border-color 0.15s, box-shadow 0.15s;
+}
+.book-chapter-link:hover {
+    border-color: var(--vl-accent);
+    box-shadow: var(--vl-shadow-sm);
+    color: var(--vl-text);
+}
+.book-chapter-num {
+    width: 1.75rem;
+    height: 1.75rem;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    background: var(--vl-accent-soft);
+    color: var(--vl-accent);
+    border-radius: 50%;
+    font-size: 0.8rem;
+    font-weight: 700;
+    flex-shrink: 0;
+}
+.book-chapter-thumb {
+    width: 56px;
+    height: 44px;
+    border-radius: 6px;
+    flex-shrink: 0;
+    background: var(--vl-accent-soft);
+    background-size: cover;
+    background-position: center;
+}
+.book-chapter-body { flex: 1; min-width: 0; }
+.book-chapter-title {
+    font-size: 0.9375rem;
+    font-weight: 600;
+    line-height: 1.3;
+    overflow: hidden;
+    white-space: nowrap;
+    text-overflow: ellipsis;
+}
+.book-chapter-meta {
+    font-size: 0.78rem;
+    color: var(--vl-muted);
+    margin-top: 0.15rem;
+}