fix:

1. 修复登录界面文字被遮挡问题
2. 大字模式关闭后显示异常问题
3. 重新调整大字模式是否开启时的字体显示效果

app-expo/src/core/typography-context.tsx

@@ -1,12 +1,14 @@
 /**
- * Typography context for large text mode (大字模式).
+ * Global typography from design tokens.
+ *
+ * - **关大字模式** → `typography.large`（舒适基准，不用过小的 `normal` 档）
+ * - **开大字模式** → `typography.xlarge`（一级页、正文等整体再放大一档）
+ *
+ * 对话气泡等仍可在页面内用 `largeText` 做更大一级的 token 选择。
  *
  * Uses React Context instead of NativeWind vars() because vars() returns empty
- * objects on native (iOS/Android), so CSS variables never cascade to children.
- * See: https://github.com/nativewind/nativewind/issues/1113
- *
- * NativeWind vars() docs (works on web only):
- * https://www.nativewind.dev/docs/api/vars
+ * objects on native (iOS/Android). See:
+ * https://github.com/nativewind/nativewind/issues/1113
  */
 import React, {
   createContext,
@@ -19,18 +21,23 @@ import { useAppSettings } from '@/hooks/use-app-settings';
 
 import tokens from '../../design-tokens.json';
 
-export type TypographyMode = 'normal' | 'large';
+export type TypographyScale = 'large' | 'xlarge';
 
 export type TypographyTokens = Record<string, number>;
 
 const TypographyContext = createContext<TypographyTokens | null>(null);
 
+const SCALE: Record<TypographyScale, TypographyTokens> = {
+  large: tokens.typography.large as TypographyTokens,
+  xlarge: tokens.typography.xlarge as TypographyTokens,
+};
+
 export function TypographyProvider({ children }: PropsWithChildren) {
   const { largeText } = useAppSettings();
-  const typography = useMemo(() => {
-    const mode: TypographyMode = largeText ? 'large' : 'normal';
-    return tokens.typography[mode] as TypographyTokens;
-  }, [largeText]);
+  const typography = useMemo(
+    () => SCALE[largeText ? 'xlarge' : 'large'],
+    [largeText],
+  );
 
   return (
     <TypographyContext.Provider value={typography}>