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/tests/core/settings/memoir-reading-settings.test.ts

@@ -0,0 +1,86 @@
+import {
+  DEFAULT_MEMOIR_READING_PREFERENCES,
+  mergeMemoirReadingPreferences,
+  parseMemoirReadingPreferences,
+} from '@/core/settings/memoir-reading-settings';
+
+describe('parseMemoirReadingPreferences', () => {
+  it('returns defaults for null and empty', () => {
+    expect(parseMemoirReadingPreferences(null)).toEqual(
+      DEFAULT_MEMOIR_READING_PREFERENCES,
+    );
+    expect(parseMemoirReadingPreferences('')).toEqual(
+      DEFAULT_MEMOIR_READING_PREFERENCES,
+    );
+    expect(parseMemoirReadingPreferences('   ')).toEqual(
+      DEFAULT_MEMOIR_READING_PREFERENCES,
+    );
+  });
+
+  it('returns defaults for invalid JSON', () => {
+    expect(parseMemoirReadingPreferences('not json')).toEqual(
+      DEFAULT_MEMOIR_READING_PREFERENCES,
+    );
+  });
+
+  it('returns defaults for non-object JSON', () => {
+    expect(parseMemoirReadingPreferences('[]')).toEqual(
+      DEFAULT_MEMOIR_READING_PREFERENCES,
+    );
+    expect(parseMemoirReadingPreferences('"x"')).toEqual(
+      DEFAULT_MEMOIR_READING_PREFERENCES,
+    );
+  });
+
+  it('fills missing or invalid fields from defaults', () => {
+    expect(parseMemoirReadingPreferences('{}')).toEqual(
+      DEFAULT_MEMOIR_READING_PREFERENCES,
+    );
+    expect(
+      parseMemoirReadingPreferences(
+        JSON.stringify({
+          fontSize: 'huge',
+          fontFamily: 'comic',
+          background: 'black',
+        }),
+      ),
+    ).toEqual(DEFAULT_MEMOIR_READING_PREFERENCES);
+  });
+
+  it('accepts valid partial object', () => {
+    expect(
+      parseMemoirReadingPreferences(
+        JSON.stringify({ fontSize: 'large' }),
+      ),
+    ).toEqual(
+      mergeMemoirReadingPreferences(DEFAULT_MEMOIR_READING_PREFERENCES, {
+        fontSize: 'large',
+      }),
+    );
+    expect(
+      parseMemoirReadingPreferences(
+        JSON.stringify({ background: 'sepia', fontFamily: 'sans' }),
+      ),
+    ).toEqual({
+      ...DEFAULT_MEMOIR_READING_PREFERENCES,
+      background: 'sepia',
+      fontFamily: 'sans',
+    });
+  });
+});
+
+describe('mergeMemoirReadingPreferences', () => {
+  it('applies only defined patch keys', () => {
+    const base = {
+      fontSize: 'small' as const,
+      fontFamily: 'sans' as const,
+      background: 'sepia' as const,
+    };
+    expect(mergeMemoirReadingPreferences(base, { fontSize: 'large' })).toEqual({
+      fontSize: 'large',
+      fontFamily: 'sans',
+      background: 'sepia',
+    });
+    expect(mergeMemoirReadingPreferences(base, {})).toEqual(base);
+  });
+});