fix: arborescence dédoublée + dossiers fermés visibles + preview pages FlowDeck

1. Tree affichage: displayTree contient maintenant les racines uniquement
   (plus de flatten). _markTree ajoute depth/db_id sans aplatir, et
   renderChildren gère la récursion. Les enfants n'apparaissent plus
   quand le dossier est fermé, et plus de dédoublement à l'ouverture.

2. Filtre: _filterTree travaille sur l'arbre hiérarchique, clone les
   nodes pour préserver la structure.

3. Preview pages FlowDeck: nouvelle API GET /api/local-workspace/page-content/{id}.
   showPreview détecte content_format='file' → /api/files/, sinon → API page-content.
   Plus de 'file not found' sur les fichiers créés dans FlowDeck.

4. API tree: ajout content_format dans la réponse pour distinguer
   fichiers uploadés vs pages FlowDeck.

app/routers/dashboard.py

@@ -554,6 +554,21 @@ async def local_workspace_tree(request: Request, folder: int = None):
             return {"tree": roots, "breadcrumb": [], "current_folder": None}
 
 
+@router.get("/api/local-workspace/page-content/{page_id:int}")
+async def get_page_content(page_id: int):
+    """Return the raw content of a page (for preview)."""
+    with get_conn() as conn:
+        row = conn.execute(
+            "SELECT content, content_format FROM pages WHERE id=?", (page_id,)
+        ).fetchone()
+    if not row:
+        return JSONResponse({"error": "Not found"}, status_code=404)
+    fmt = row["content_format"]
+    if fmt == "file":
+        return JSONResponse({"content": "(uploaded file)", "format": fmt})
+    return {"content": row["content"] or "", "format": fmt}
+
+
 @router.get("/api/local-workspace/breadcrumb")
 async def local_workspace_breadcrumb(request: Request, folder: int):
     """Return breadcrumb trail for a folder."""
@@ -566,13 +581,13 @@ def _build_tree_children(conn, parent_id: int | None, ws_id: int) -> list:
     """Recursively build the tree of children for a node."""
     if parent_id is None:
         rows = conn.execute(
-            "SELECT id, title, parent_section FROM pages "
+            "SELECT id, title, parent_section, content_format FROM pages "
             "WHERE parent_id IS NULL AND workspace_id=? ORDER BY created_at DESC",
             (ws_id,),
         ).fetchall()
     else:
         rows = conn.execute(
-            "SELECT id, title, parent_section FROM pages "
+            "SELECT id, title, parent_section, content_format FROM pages "
             "WHERE parent_id=? AND workspace_id=? ORDER BY created_at DESC",
             (parent_id, ws_id),
         ).fetchall()
@@ -586,6 +601,7 @@ def _build_tree_children(conn, parent_id: int | None, ws_id: int) -> list:
             "name": r["title"] or "Untitled",
             "type": "folder" if is_folder else "page",
             "is_folder": is_folder,
+            "content_format": r["content_format"] if not is_folder else None,
             "children": children,
             "has_children": len(children) > 0,
         })