feat(export): v4.7.0 Export — Markdown, PDF, HTML, Site statique

- Service serveur app/services/export.py: conversion blocs vers Markdown / HTML / PDF
- 4 endpoints: /api/export/markdown|html|pdf|site/{page_id}
- Export Markdown complet (tous les blocs + images + sous-pages recursives)
- Export PDF via xhtml2pdf (pur Python, aucune lib systeme)
- Export HTML standalone self-contained (styles inline)
- Export Site: zip multi-pages (index.html + une page par sous-page)
- UI: menu 'More > Export' dans l'editeur (Markdown, HTML, PDF, Site .zip)
- Tests: 190 passing (6 nouveaux)

static/css/app.css

@@ -3212,6 +3212,15 @@ button.breadcrumb-current { max-width: 260px; }
 .more-menu-item.danger { color: var(--danger); }
 .more-menu-sep { height: 1px; background: var(--border); margin: 4px 0; }
 
+/* ── More menu sub-section (e.g. Export formats) ── */
+.more-menu-sub {
+  border-top: 1px solid var(--border);
+  border-bottom: 1px solid var(--border);
+  background: var(--bg-secondary);
+  margin: 2px 0; padding: 2px 0 2px 6px;
+}
+.more-menu-sub .more-menu-item { font-size: 12.5px; padding: 6px 12px; }
+
 /* ── Toggle switch ── */
 .toggle-switch { position: relative; display: inline-block; width: 36px; height: 20px; }
 .toggle-switch.sm { width: 32px; height: 18px; }