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/features/memoir/markdown-renderer.tsx

@@ -9,6 +9,10 @@ import React from 'react';
 import { Platform, StyleSheet, Text, View } from 'react-native';
 import Markdown from 'react-native-markdown-display';
 
+import type {
+  MemoirReadingFontFamily,
+  MemoirReadingFontSize,
+} from '@/core/settings/memoir-reading-settings';
 import type { ImageAsset } from './types';
 
 const PLACEHOLDER_RE = /\{\{\{\{IMAGE:(.*?)\}\}\}\}|\{\{IMAGE:(.*?)\}\}/g;
@@ -197,8 +201,8 @@ export interface MarkdownRendererProps {
   markdown: string;
   renderedAssets: ImageAsset[];
   coverImageUrl: string | null;
-  fontSize: 'small' | 'default' | 'large';
-  fontFamily: 'serif' | 'sans';
+  fontSize: MemoirReadingFontSize;
+  fontFamily: MemoirReadingFontFamily;
   backgroundColor: string;
   contentWidth: number;
   /** 多故事分段时仅首段下沉首字 */