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/dashboard.py

@@ -264,11 +264,16 @@ async def view_page_root(request: Request, page_id: int):
         page_data["file_size"] = file_size
         page_data["file_name"] = filename
 
+    with get_conn() as conn:
+        nav_crumbs = _nav_breadcrumb(conn, page_id)
     ctx = {**sidebar, "page": page, "sub_pages": [dict(s) for s in subs],
            "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"})
@@ -827,6 +832,99 @@ def _build_breadcrumb(conn, folder_id: int) -> list:
     return breadcrumb
 
 
+def _nav_breadcrumb(conn, page_id: int) -> list:
+    """Build a Notion-style breadcrumb chain (root -> page) for the header.
+
+    Returns a list of dicts: {id, label, url, icon, menu}. The last item is the
+    current page (url = None). Every item has ``menu: True`` so the header can
+    open a sibling-navigation dropdown for it.
+    """
+    from app.routers.board import _file_icon
+    chain = []
+    current = page_id
+    seen = set()
+    while current and current not in seen:
+        seen.add(current)
+        row = conn.execute(
+            "SELECT id, title, parent_id, parent_section, content_format "
+            "FROM pages WHERE id=? AND deleted_at IS NULL",
+            (current,),
+        ).fetchone()
+        if not row:
+            break
+        is_folder = row["parent_section"] == "Workspace"
+        title = row["title"] or "Untitled"
+        chain.insert(0, {
+            "id": row["id"],
+            "label": title,
+            "url": None,
+            "icon": "\U0001F4C1" if is_folder else _file_icon(title, row["content_format"]),
+            "menu": True,
+        })
+        current = row["parent_id"]
+    # All items except the current page are navigable links.
+    for i, item in enumerate(chain):
+        if i < len(chain) - 1:
+            item["url"] = f"/pages/{item['id']}"
+    return chain
+
+
+@router.get("/api/nav/menu")
+async def nav_menu(request: Request, workspace_id: int = None, parent_id: int = None):
+    """Return the pages at one level for the header breadcrumb navigation menu.
+
+    If ``parent_id`` is given, returns that page's children; otherwise the
+    workspace's root pages. Each item includes ``has_children`` so the frontend
+    can render an expandable sub-menu.
+    """
+    from app.routers.board import _file_icon
+    ws_id = workspace_id
+    if not ws_id:
+        ws = _get_active_workspace(request, user_id=_get_user_id(request))
+        ws_id = ws["id"] if ws else None
+    if not ws_id:
+        return {"items": []}
+    with get_conn() as conn:
+        if parent_id:
+            rows = conn.execute(
+                "SELECT id, title, parent_section, content_format FROM pages "
+                "WHERE parent_id=? AND workspace_id=? AND deleted_at IS NULL "
+                "ORDER BY sort_order ASC, created_at DESC",
+                (parent_id, ws_id),
+            ).fetchall()
+        else:
+            rows = conn.execute(
+                "SELECT id, title, parent_section, content_format FROM pages "
+                "WHERE parent_id IS NULL AND workspace_id=? AND deleted_at IS NULL "
+                "ORDER BY sort_order ASC, created_at DESC",
+                (ws_id,),
+            ).fetchall()
+        ids = [r["id"] for r in rows]
+        child_counts = {}
+        if ids:
+            placeholders = ",".join("?" for _ in ids)
+            cc_rows = conn.execute(
+                f"SELECT parent_id, COUNT(*) AS c FROM pages "
+                f"WHERE parent_id IN ({placeholders}) AND deleted_at IS NULL "
+                f"GROUP BY parent_id",
+                ids,
+            ).fetchall()
+            for cr in cc_rows:
+                child_counts[cr["parent_id"]] = cr["c"]
+        items = []
+        for r in rows:
+            is_folder = r["parent_section"] == "Workspace"
+            title = r["title"] or "Untitled"
+            items.append({
+                "id": r["id"],
+                "name": title,
+                "icon": "\U0001F4C1" if is_folder else _file_icon(title, r["content_format"]),
+                "has_children": child_counts.get(r["id"], 0) > 0,
+                "url": f"/pages/{r['id']}",
+            })
+    return {"items": items}
+
+
 @router.post("/api/local-workspace/items")
 async def create_local_workspace_item(request: Request):
     """Create a new file in the active workspace."""