feat(narrative): add frontend API client

Five named exports covering narrative endpoints, following the
existing pattern in frontend/src/api/simulation.js (service +
requestWithRetry + named exports).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/api/narrative.js

@@ -0,0 +1,42 @@
+import service, { requestWithRetry } from './index'
+
+/**
+ * Get the full generated story for a simulation.
+ * @param {string} simId
+ */
+export const getFullStory = (simId) => {
+  return service.get(`/api/narrative/story/${simId}`)
+}
+
+/**
+ * Get a single round's story beat.
+ * @param {string} simId
+ * @param {number} roundNum
+ */
+export const getRoundStory = (simId, roundNum) => {
+  return service.get(`/api/narrative/story/${simId}/round/${roundNum}`)
+}
+
+/**
+ * Translate a round on demand — generates prose via LLM and stores the beat.
+ * @param {Object} data - { sim_id, round, platform?, tone? }
+ */
+export const translateRound = (data) => {
+  return requestWithRetry(() => service.post('/api/narrative/translate', data), 3, 2000)
+}
+
+/**
+ * Get extended character roster with emotional state.
+ * @param {string} simId
+ */
+export const getCharacters = (simId) => {
+  return service.get(`/api/narrative/characters/${simId}`)
+}
+
+/**
+ * Bootstrap narrative character profiles from existing OASIS profiles.
+ * @param {string} simId
+ */
+export const initCharacters = (simId) => {
+  return requestWithRetry(() => service.post(`/api/narrative/characters/${simId}/init`), 3, 1000)
+}