feat(08-07): create domain modules documents.js, auth.js, topics.js — CODE-04

- documents.js: 10 functions; fetchDocumentContent refactored to use fetchWithRetry
- auth.js: 15 functions (login through getMyQuota) — import request from utils.js
- topics.js: 5 functions — import request from utils.js
- All functions copied verbatim from client.js; no consumer files modified

frontend/src/api/topics.js

@@ -0,0 +1,39 @@
+/**
+ * Topics API — listing, creation, update, deletion, AI suggestion.
+ *
+ * Consumers: stores/topics.js
+ */
+
+import { request } from './utils.js'
+
+export function listTopics() {
+  return request('/api/topics')
+}
+
+export function createTopic({ name, description = '', color = '#6366f1' }) {
+  return request('/api/topics', {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify({ name, description, color }),
+  })
+}
+
+export function updateTopic(id, patch) {
+  return request(`/api/topics/${id}`, {
+    method: 'PATCH',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify(patch),
+  })
+}
+
+export function deleteTopic(id) {
+  return request(`/api/topics/${id}`, { method: 'DELETE' })
+}
+
+export function suggestTopics(documentId) {
+  return request('/api/topics/suggest', {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify({ document_id: documentId }),
+  })
+}