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';

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

@@ -0,0 +1,70 @@
+export type MemoirReadingFontSize = 'small' | 'default' | 'large';
+
+export type MemoirReadingFontFamily = 'serif' | 'sans';
+
+export type MemoirReadingBackground = 'white' | 'sepia';
+
+export interface MemoirReadingPreferences {
+  fontSize: MemoirReadingFontSize;
+  fontFamily: MemoirReadingFontFamily;
+  background: MemoirReadingBackground;
+}
+
+export const DEFAULT_MEMOIR_READING_PREFERENCES: MemoirReadingPreferences = {
+  fontSize: 'default',
+  fontFamily: 'serif',
+  background: 'white',
+};
+
+const FONT_SIZES: MemoirReadingFontSize[] = ['small', 'default', 'large'];
+const FONT_FAMS: MemoirReadingFontFamily[] = ['serif', 'sans'];
+const BACKGROUNDS: MemoirReadingBackground[] = ['white', 'sepia'];
+
+function isFontSize(v: unknown): v is MemoirReadingFontSize {
+  return typeof v === 'string' && FONT_SIZES.includes(v as MemoirReadingFontSize);
+}
+
+function isFontFamily(v: unknown): v is MemoirReadingFontFamily {
+  return typeof v === 'string' && FONT_FAMS.includes(v as MemoirReadingFontFamily);
+}
+
+function isBackground(v: unknown): v is MemoirReadingBackground {
+  return typeof v === 'string' && BACKGROUNDS.includes(v as MemoirReadingBackground);
+}
+
+export function parseMemoirReadingPreferences(
+  raw: string | null,
+): MemoirReadingPreferences {
+  if (raw == null || raw.trim() === '') {
+    return { ...DEFAULT_MEMOIR_READING_PREFERENCES };
+  }
+  try {
+    const parsed: unknown = JSON.parse(raw);
+    if (parsed == null || typeof parsed !== 'object' || Array.isArray(parsed)) {
+      return { ...DEFAULT_MEMOIR_READING_PREFERENCES };
+    }
+    const o = parsed as Record<string, unknown>;
+    const d = DEFAULT_MEMOIR_READING_PREFERENCES;
+    return {
+      fontSize: isFontSize(o.fontSize) ? o.fontSize : d.fontSize,
+      fontFamily: isFontFamily(o.fontFamily) ? o.fontFamily : d.fontFamily,
+      background: isBackground(o.background) ? o.background : d.background,
+    };
+  } catch {
+    return { ...DEFAULT_MEMOIR_READING_PREFERENCES };
+  }
+}
+
+export function mergeMemoirReadingPreferences(
+  base: MemoirReadingPreferences,
+  patch: Partial<MemoirReadingPreferences>,
+): MemoirReadingPreferences {
+  return {
+    fontSize:
+      patch.fontSize !== undefined ? patch.fontSize : base.fontSize,
+    fontFamily:
+      patch.fontFamily !== undefined ? patch.fontFamily : base.fontFamily,
+    background:
+      patch.background !== undefined ? patch.background : base.background,
+  };
+}