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/templates/workspaces.html

@@ -4,7 +4,7 @@
 {% block page_icon %}🏠{% endblock %}
 
 {% block topbar %}
-{% set breadcrumb_items = [{"label": "Home", "url": "/workspaces"}, {"label": "Workspaces", "url": None}] %}
+{% set breadcrumb_items = [{"label": "Workspaces", "url": None}] %}
 {% set page_icon = "🏠" %}
 {% set right_actions = '<a href="/auth/login?provider=local" class="topbar-btn" title="Login">🔑</a><a href="/accounts/settings" class="topbar-btn" title="Settings">⚙</a>' %}
 {% include '_header.html' %}