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/providers.tsx

@@ -2,6 +2,7 @@ import React, { type PropsWithChildren } from 'react';
 
 import { initApiClient } from '@/core/api/client';
 import { AppSettingsProvider } from '@/core/app-settings-context';
+import { MemoirReadingSettingsProvider } from '@/core/memoir-reading-settings-context';
 import { NetworkError } from '@/core/api/types';
 import { tokenManager } from '@/core/auth/token-manager';
 import { config } from '@/core/config';
@@ -62,7 +63,9 @@ initApiClient({
 export function AppProviders({ children }: PropsWithChildren) {
   return (
     <AppQueryProvider>
-      <AppSettingsProvider>{children}</AppSettingsProvider>
+      <AppSettingsProvider>
+        <MemoirReadingSettingsProvider>{children}</MemoirReadingSettingsProvider>
+      </AppSettingsProvider>
     </AppQueryProvider>
   );
 }