feat: add Notion-style breadcrumb navigation with dropdown menus

Backend:
- Added _nav_breadcrumb() to build page hierarchy chain (root → current)
- New /api/nav/menu endpoint returns workspace/folder children with has_children flag
- view_page() and view_page_root() now pass breadcrumb_items, nav_workspace_id, nav_page_id to template

Header template (_header.html):
- Replaced static breadcrumb with Alpine.js fdBreadcrumb() component
- Breadcrumb items now clickable with dropdown menus showing

app/routers/board.py

@@ -965,10 +965,16 @@ async def view_page(request: Request, page_id: int):
         page_data["file_size"] = file_size
         page_data["file_name"] = filename
 
+    from app.routers.dashboard import _nav_breadcrumb
+    with get_conn() as conn:
+        nav_crumbs = _nav_breadcrumb(conn, page_id)
     ctx = {**sidebar, "page": page, "page_favorited": fav is not None,
            "page_share_mode": page.get("share_mode", "private"),
            "page_published": bool(page.get("published", 0)),
-           "page_data": page_data}
+           "page_data": page_data,
+           "breadcrumb_items": nav_crumbs,
+           "nav_workspace_id": page.get("workspace_id") or 0,
+           "nav_page_id": page_id}
     template = env.get_template("page_editor.html")
     response = template.render(**ctx)
     return HTMLResponse(content=response, headers={"Cache-Control": "no-store, max-age=0"})