fix: chargement tree sidebar 100% client-side (pas de asyncio.run)

Avant: _sidebar_data() utilisait asyncio.run() → RuntimeWarning + 302 redirects
Après: gitea_workspace.html appelle loadSidebarTree() → fetch API → injecte dans #sidebar-workspace-items

- base.html: ajout id='sidebar-workspace-items' sur le <ul> workspace
- dashboard.py: _sidebar_data() ne fait plus de fetch serveur, juste passe owner/repo

app/templates/base.html

@@ -167,7 +167,7 @@
   {% endfor %}
 {% endmacro %}
       <div class="sidebar-section-items" x-show="sectionsOpen.workspace" x-transition>
-        <ul class="sidebar-items" data-section="workspace">
+        <ul class="sidebar-items" data-section="workspace" id="sidebar-workspace-items">
           {% if workspace_pages %}
             {{ render_workspace_tree(workspace_pages) }}
           {% else %}