feat(app-expo): tiered large-text presets with English-friendly default

Replace the boolean large-text flag with three global typography tiers,
defaulting new installs to the smallest tier when English is in effect
while preserving legacy storage and Chinese defaults. Add a profile
sub-screen to pick the tier and unit tests for storage resolution.

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

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

@@ -1,3 +1,4 @@
+import { getLocales } from 'expo-localization';
 import { Platform } from 'react-native';
 
 import {
@@ -6,7 +7,11 @@ import {
   setSecureItem,
 } from '@/core/storage/secure';
 
-import { supportedLanguages, type AppLanguage } from '@/i18n/resources';
+import {
+  fallbackLanguage,
+  supportedLanguages,
+  type AppLanguage,
+} from '@/i18n/resources';
 
 import { THEME_NAMES, type ThemeName } from '@/constants/theme-bridge';
 
@@ -18,6 +23,7 @@ import {
 
 const KEY_LANGUAGE = 'app_settings_language';
 const KEY_LARGE_TEXT = 'app_settings_large_text';
+const KEY_LARGE_TEXT_LEVEL = 'app_settings_large_text_level';
 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';
@@ -62,14 +68,73 @@ export async function clearAppLanguage(): Promise<void> {
   await deleteStored(KEY_LANGUAGE);
 }
 
-export async function getLargeText(): Promise<boolean> {
-  const v = await getStored(KEY_LARGE_TEXT);
-  if (v == null || v === '') return true;
-  return v === 'true';
+export type LargeTextLevel = 0 | 1 | 2;
+
+function parseStoredLargeTextLevel(raw: string | null): LargeTextLevel | null {
+  if (raw === '0' || raw === '1' || raw === '2') {
+    return Number(raw) as LargeTextLevel;
+  }
+  return null;
 }
 
-export async function setLargeText(value: boolean): Promise<void> {
-  await setStored(KEY_LARGE_TEXT, value ? 'true' : 'false');
+function deviceLanguageFromLocales(): AppLanguage {
+  const locale = getLocales()[0];
+  const tag = locale?.languageCode ?? locale?.languageTag;
+  if (!tag) return fallbackLanguage;
+  return tag.toLowerCase().startsWith('zh') ? 'zh' : 'en';
+}
+
+async function effectiveAppLanguage(): Promise<AppLanguage> {
+  const override = await getAppLanguage();
+  return override ?? deviceLanguageFromLocales();
+}
+
+/** 无存储时的默认档位：英文关大字、中文沿用偏大默认 */
+export function defaultLargeTextLevelForLanguage(lang: AppLanguage): LargeTextLevel {
+  return lang === 'en' ? 0 : 1;
+}
+
+/**
+ * 纯函数：由原始存储值与当前生效语言解析档位（供单测与实现共用）。
+ */
+export function computeLargeTextLevelFromStorage(
+  levelRaw: string | null,
+  legacyLargeTextRaw: string | null,
+  effectiveLanguage: AppLanguage,
+): LargeTextLevel {
+  const parsed = parseStoredLargeTextLevel(levelRaw);
+  if (parsed !== null) return parsed;
+  if (legacyLargeTextRaw === 'true') return 1;
+  if (legacyLargeTextRaw === 'false') return 0;
+  return defaultLargeTextLevelForLanguage(effectiveLanguage);
+}
+
+export async function getLargeTextLevel(): Promise<LargeTextLevel> {
+  const [levelRaw, legacyRaw, lang] = await Promise.all([
+    getStored(KEY_LARGE_TEXT_LEVEL),
+    getStored(KEY_LARGE_TEXT),
+    effectiveAppLanguage(),
+  ]);
+  const level = computeLargeTextLevelFromStorage(levelRaw, legacyRaw, lang);
+
+  const hasValidLevel = parseStoredLargeTextLevel(levelRaw) !== null;
+  if (hasValidLevel && legacyRaw != null && legacyRaw !== '') {
+    await deleteStored(KEY_LARGE_TEXT);
+  } else if (
+    !hasValidLevel &&
+    legacyRaw != null &&
+    legacyRaw !== ''
+  ) {
+    await setStored(KEY_LARGE_TEXT_LEVEL, String(level));
+    await deleteStored(KEY_LARGE_TEXT);
+  }
+
+  return level;
+}
+
+export async function setLargeTextLevel(level: LargeTextLevel): Promise<void> {
+  await setStored(KEY_LARGE_TEXT_LEVEL, String(level));
+  await deleteStored(KEY_LARGE_TEXT);
 }
 
 export async function getDarkMode(): Promise<boolean> {