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

static/css/app.css

@@ -311,11 +311,58 @@ input, select, textarea { font-family: inherit; color: inherit; }
   background: var(--bg-active);
 }
 
+/* Tree toggle button (▶/▼ chevron) */
+.tree-toggle {
+  width: 18px;
+  height: 18px;
+  border-radius: 3px;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  color: var(--text-secondary);
+  font-size: 9px;
+  flex-shrink: 0;
+  transition: transform 0.15s ease, background var(--transition);
+  padding: 0;
+  margin: 0;
+  background: none;
+  border: none;
+}
+
+.tree-toggle:hover {
+  background: var(--bg-tertiary);
+  color: var(--text-primary);
+}
+
+.tree-toggle.expanded {
+  transform: rotate(90deg);
+}
+
+.tree-toggle-placeholder {
+  width: 18px;
+  height: 18px;
+  flex-shrink: 0;
+}
+
+/* Tree children wrapper */
+.tree-children-wrapper {
+  list-style: none;
+}
+
+.tree-children-wrapper .sidebar-items {
+  padding-left: 0;
+}
+
 .sidebar-item .page-icon {
   font-size: 14px;
   width: 18px;
   text-align: center;
   flex-shrink: 0;
+  cursor: grab;
+}
+
+.sidebar-item:active .page-icon {
+  cursor: grabbing;
 }
 
 .sidebar-item .page-name {
@@ -353,10 +400,6 @@ input, select, textarea { font-family: inherit; color: inherit; }
   background: var(--bg-tertiary);
 }
 
-/* Indented sub-items */
-.sidebar-item.indent-1 { padding-left: 24px; }
-.sidebar-item.indent-2 { padding-left: 40px; }
-
 /* Sidebar footer */
 .sidebar-footer {
   margin-top: auto;