feat: arborescence interactive avec expand/collapse, breadcrumb et navigation par dossier

- API /api/local-workspace/tree: arbre récursif complet (support optionnel ?folder=ID)
- API /api/local-workspace/breadcrumb: fil d'Ariane parent pour un dossier
- Page /local-workspace: supporte ?folder=ID pour naviguer dans un dossier
- Sidebar: expand/collapse (▶/▼) sur les dossiers, clic dossier → /local-workspace?folder=ID
- CSS: styles chevron .tree-chevron, .tree-folder-link
- Alpine.js: expandedFolders, toggleTreeFolder, navigateToFolder

static/css/app.css

@@ -397,8 +397,51 @@ input, select, textarea { font-family: inherit; color: inherit; }
 .sidebar-item .page-name {
   flex: 1;
   overflow: hidden;
-  text-overflow: ellipsis;
   white-space: nowrap;
+  text-overflow: ellipsis;
+  color: var(--text);
+  text-decoration: none;
+  font-size: 13px;
+}
+
+/* Tree chevron for expand/collapse */
+.tree-chevron {
+  width: 16px;
+  height: 16px;
+  flex-shrink: 0;
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
+  background: none;
+  border: none;
+  color: var(--text-tertiary);
+  cursor: pointer;
+  font-size: 8px;
+  padding: 0;
+  border-radius: 3px;
+  transition: transform 150ms, color 100ms;
+}
+.tree-chevron:hover {
+  color: var(--text);
+  background: var(--bg-hover);
+}
+.tree-chevron.open {
+  transform: rotate(90deg);
+}
+
+/* Folder link styling */
+.tree-folder-link {
+  flex: 1;
+  overflow: hidden;
+  white-space: nowrap;
+  text-overflow: ellipsis;
+  color: var(--text);
+  font-size: 13px;
+  cursor: pointer;
+  font-weight: 500;
+}
+.tree-folder-link:hover {
+  color: var(--accent);
 }
 
 .sidebar-item .item-actions {