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)

app/templates/_page_editor_content.html

@@ -345,7 +345,15 @@
                 @click.outside="!_justOpened && (moreOpen = false)"
                 x-transition
             >
-                <div class="more-menu-item" @click="exportPage">↗ Export</div>
+                <div class="more-menu-item" @click="moreOpen = false; exportOpen = true">↗ Export</div>
+                <template x-if="exportOpen">
+                  <div class="more-menu-sub">
+                    <div class="more-menu-item" @click="exportPage('markdown')"> Markdown (.md)</div>
+                    <div class="more-menu-item" @click="exportPage('html')"> HTML (.html)</div>
+                    <div class="more-menu-item" @click="exportPage('pdf')"> PDF (.pdf)</div>
+                    <div class="more-menu-item" @click="exportPage('site')"> Site statique (.zip)</div>
+                  </div>
+                </template>
                 <div class="more-menu-item" @click="duplicatePage">
                     {{ fd_icon("copy",14) }} Duplicate
                 </div>