feat(memoir): persist chapter reading prefs globally

Share font size, font family, and background across all memoir chapters via MemoirReadingSettingsProvider and SecureStore (same app-settings pattern). Add parse/merge helpers and unit tests.

Co-authored-by: Cursor <cursoragent@cursor.com>

app-expo/src/core/settings/app-settings.ts

@@ -10,11 +10,18 @@ import { supportedLanguages, type AppLanguage } from '@/i18n/resources';
 
 import { THEME_NAMES, type ThemeName } from '@/constants/theme-bridge';
 
+import {
+  mergeMemoirReadingPreferences,
+  parseMemoirReadingPreferences,
+  type MemoirReadingPreferences,
+} from './memoir-reading-settings';
+
 const KEY_LANGUAGE = 'app_settings_language';
 const KEY_LARGE_TEXT = 'app_settings_large_text';
 const KEY_DARK_MODE = 'app_settings_dark_mode';
 const KEY_THEME_NAME = 'app_settings_theme_name';
 const KEY_TTS_SPEAK_DEFAULT = 'app_settings_tts_speak_default';
+const KEY_MEMOIR_READING = 'app_settings_memoir_reading';
 
 const webFallback: Record<string, string> = {};
 
@@ -95,5 +102,20 @@ export async function setTtsSpeakDefault(value: boolean): Promise<void> {
   await setStored(KEY_TTS_SPEAK_DEFAULT, value ? 'true' : 'false');
 }
 
+export async function getMemoirReadingPreferences(): Promise<MemoirReadingPreferences> {
+  const raw = await getStored(KEY_MEMOIR_READING);
+  return parseMemoirReadingPreferences(raw);
+}
+
+export async function setMemoirReadingPreferences(
+  patch: Partial<MemoirReadingPreferences>,
+): Promise<void> {
+  const raw = await getStored(KEY_MEMOIR_READING);
+  const current = parseMemoirReadingPreferences(raw);
+  const next = mergeMemoirReadingPreferences(current, patch);
+  await setStored(KEY_MEMOIR_READING, JSON.stringify(next));
+}
+
 export { supportedLanguages, THEME_NAMES };
 export type { AppLanguage, ThemeName };
+export type { MemoirReadingPreferences } from './memoir-reading-settings';