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/gitea_workspace.html

@@ -4,7 +4,7 @@
 {% block page_icon %}🔗{% endblock %}
 
 {% block topbar %}
-{% set breadcrumb_items = [{"label": "Home", "url": "/workspaces"}, {"label": owner ~ "/" ~ repo, "url": None}] %}
+{% set breadcrumb_items = [{"label": owner ~ "/" ~ repo, "url": None}] %}
 {% set page_icon = "🔗" %}
 {% set right_actions = '<button class="page-action-btn" @click="showNewFile=!showNewFile" title="New file">📄+</button><button class="page-action-btn" @click="triggerUpload()" title="Upload file">📤</button><input type="file" id="gitea-upload-input" style="display:none" @change="doUpload($event)" multiple><button class="page-action-btn" @click="refreshTree()" title="Refresh">🔄</button><a href="/accounts/settings" class="topbar-btn" title="Settings">⚙</a>' %}
 {% include '_header.html' %}