feat: sidebar tree navigation — toggle chevrons + drag & drop

- Add sort_order column to pages table for drag & drop reordering
- Rewrite _sidebar_data with recursive _build_page_tree for nested hierarchy
- Add tree toggle buttons (▶ collapsed / ▼ expanded) on parent items
- Alpine.js expandedNodes state with toggleTreeNode() for expand/collapse
- Jinja2 recursive macro render_tree_item for proper nesting + indentation
- SortableJS integration on .sidebar-items for drag & drop repositioning
- New PUT /board/api/pages/{id}/move API for persisting moves
- Auto-set sort_order on page creation (max+1 for same parent)
- CSS: .tree-toggle, .tree-toggle-placeholder, grab cursor on page-icon
- 28/28 tests pass, all endpoints healthy

app/routers/dashboard.py

@@ -29,6 +29,9 @@ def _sidebar_data(request: Request, repos: list[dict]) -> dict:
             "url": f"/board/{full_name}",
             "active": False,
             "indent": 0,
+            "depth": 0,
+            "has_children": False,
+            "children": [],
         })
 
     return {