fix: resolve API inconsistencies found by Mokoto

- Bug 1: No fix needed - frontend already uses PUT for document updates
- Bug 2: Changed folders API to use 'project' param (matches documents)
- Bug 3: GET /folders now works without project filter (lists all folders)

Changes:
- folders.js: Accept 'project' instead of 'projectId', make it optional
- folderService.js: Support listing all folders when projectId is null
- api.js: Updated getFolders() to use 'project' param consistently

public/js/api.js

@@ -171,10 +171,12 @@ class ApiClient {
   }
 
   // ===== Folders =====
-  getFolders(projectId, parentId = null) {
-    const params = new URLSearchParams({ projectId });
+  getFolders(project = null, parentId = null) {
+    const params = new URLSearchParams();
+    if (project) params.append('project', project);
     if (parentId) params.append('parentId', parentId);
-    return this.get(`/folders?${params.toString()}`);
+    const query = params.toString();
+    return this.get(`/folders${query ? '?' + query : ''}`);
   }
 
   getFolder(id) {