fix:

1. 修复安卓部分机型顶部安全区遮挡回忆录标题的问题；
2. 降低封面图生成阈值和展示逻辑，独立封面图未生成时，使用正文图；
3. 去掉“嗯。”生硬回答，去掉不合理段首承接词；
4. 新增章节封面所需最少插图数的配置项

app-expo/src/app/(main)/chapter/[id].tsx

@@ -18,8 +18,13 @@ import { useTranslation } from 'react-i18next';
 
 import { Icon } from '@/components/ui/icon';
 import { Text } from '@/components/ui/text';
-import { ScreenHeader } from '@/components/screen-header';
+import {
+  getScreenHeaderLayoutMetrics,
+  ScreenHeader,
+} from '@/components/screen-header';
 import { ScreenGutter } from '@/constants/layout';
+import { useTypography } from '@/core/typography-context';
+import { useAppSettings } from '@/hooks/use-app-settings';
 import {
   MarkdownRenderer,
   ReadingMarkdownHorizontalRuleInColumn,
@@ -353,6 +358,8 @@ function ReadingSettingsModal({
 export default function ChapterScreen() {
   const { id } = useLocalSearchParams<{ id: string }>();
   const insets = useSafeAreaInsets();
+  const { largeText } = useAppSettings();
+  const typography = useTypography();
   const { width } = useWindowDimensions();
   const { t } = useTranslation('memoir');
   const { data: chapter, isLoading } = useChapterDetail(id ?? '');
@@ -429,8 +436,13 @@ export default function ChapterScreen() {
   const useReadingSegments =
     Array.isArray(readingSegments) && readingSegments.length > 0;
 
-  /** 与 ScreenHeader（reading、useSafeArea）可视高度对齐，避免返回栏与首屏内容之间出现空隙 */
-  const headerOccupiedHeight = Math.max(insets.top, 12) + 56;
+  /** 与 ScreenHeader（reading、useSafeArea）实际总高度一致，避免章节标题被顶栏或安全区遮挡 */
+  const headerOccupiedHeight = getScreenHeaderLayoutMetrics(insets, {
+    useSafeArea: true,
+    variant: 'reading',
+    largeText,
+    typography,
+  }).totalHeight;
 
   const handleDeletePress = () => {
     Alert.alert(

app-expo/src/components/screen-header.tsx

@@ -9,6 +9,8 @@ import { useTypography } from '@/core/typography-context';
 import { ScreenGutter } from '@/constants/layout';
 import { useAppSettings } from '@/hooks/use-app-settings';
 
+import type { TypographyTokens } from '@/core/typography-context';
+
 /** 默认最小触控目标 48dp；大字模式下与标题字号匹配，略向左扩展便于够到边缘 */
 const BACK_HIT_MIN = 48;
 const BACK_HIT_MIN_LARGE = 56;
@@ -18,6 +20,52 @@ const BACK_EXTRA_HIT_LEFT = 4;
 
 export type ScreenHeaderVariant = 'default' | 'chat' | 'reading';
 
+export type ScreenHeaderLayoutOpts = {
+  useSafeArea: boolean;
+  variant: ScreenHeaderVariant;
+  largeText: boolean;
+  typography: TypographyTokens;
+};
+
+/**
+ * 与组件内布局一致的总高度（含顶部安全区内边距），供绝对定位顶栏下的 ScrollView paddingTop 等使用。
+ */
+export function getScreenHeaderLayoutMetrics(
+  insets: { top: number },
+  opts: ScreenHeaderLayoutOpts,
+) {
+  const barPaddingBottom = opts.largeText ? 18 : 16;
+  const titleRowPaddingV = opts.largeText ? 8 : 4;
+  const backTouchMin = opts.largeText ? BACK_HIT_MIN_LARGE : BACK_HIT_MIN;
+  const { variant, typography, largeText } = opts;
+  const titleFontSize =
+    variant === 'chat'
+      ? largeText
+        ? typography.headingMedium
+        : typography.headingSmall
+      : Math.max(typography.titleLarge, typography.headingSmall);
+  const titleLineMin =
+    variant === 'chat'
+      ? Math.ceil(titleFontSize * (largeText ? 1.45 : 1.38)) +
+        (largeText ? 14 : 10)
+      : Math.ceil(titleFontSize * 1.32) + (largeText ? 10 : 8);
+  const titleRowMinHeight = Math.max(backTouchMin, titleLineMin);
+  const titleRowOuterHeight = titleRowMinHeight + 2 * titleRowPaddingV;
+  const paddingTop = opts.useSafeArea ? Math.max(insets.top, 12) : 12;
+  const totalHeight = paddingTop + titleRowOuterHeight + barPaddingBottom;
+  return {
+    barPaddingBottom,
+    titleRowPaddingV,
+    titleFontSize,
+    titleLineMin,
+    titleRowMinHeight,
+    titleRowOuterHeight,
+    paddingTop,
+    totalHeight,
+    backTouchMin,
+  };
+}
+
 const VARIANT_COLORS = {
   default: {
     title: undefined, // use theme foreground
@@ -76,7 +124,6 @@ export function ScreenHeader({
   const typography = useTypography();
   const colors = VARIANT_COLORS[variant];
 
-  const backTouchMin = largeText ? BACK_HIT_MIN_LARGE : BACK_HIT_MIN;
   const backIconSize = largeText ? BACK_ICON_SIZE_LARGE : BACK_ICON_SIZE;
 
   const handleBack = onBack ?? (() => router.back());
@@ -88,27 +135,26 @@ export function ScreenHeader({
    * 不要用外层 minHeight「框死」整块栏：paddingTop 含安全区时会把内容区压扁。
    * 标题行最小高度随当前排版 token 计算（关大字也要留够，避免裁字 / Android font padding）。
    */
-  const barPaddingBottom = largeText ? 18 : 16;
-  const titleRowPaddingV = largeText ? 8 : 4;
-  const titleFontSize =
-    variant === 'chat'
-      ? largeText
-        ? typography.headingMedium
-        : typography.headingSmall
-      : Math.max(typography.titleLarge, typography.headingSmall);
-  const titleLineMin =
-    variant === 'chat'
-      ? Math.ceil(titleFontSize * (largeText ? 1.45 : 1.38)) +
-        (largeText ? 14 : 10)
-      : Math.ceil(titleFontSize * 1.32) + (largeText ? 10 : 8);
-  const titleRowMinHeight = Math.max(backTouchMin, titleLineMin);
+  const {
+    barPaddingBottom,
+    titleRowPaddingV,
+    titleFontSize,
+    titleRowMinHeight,
+    paddingTop: headerPaddingTop,
+    backTouchMin,
+  } = getScreenHeaderLayoutMetrics(insets, {
+    useSafeArea,
+    variant,
+    largeText,
+    typography,
+  });
 
   const containerStyle = {
     flexDirection: 'row' as const,
     alignItems: 'center' as const,
     justifyContent: 'space-between' as const,
     paddingHorizontal: Math.max(ScreenGutter, 16),
-    paddingTop: useSafeArea ? Math.max(insets.top, 12) : 12,
+    paddingTop: headerPaddingTop,
     paddingBottom: barPaddingBottom,
     ...(bgColor && { backgroundColor: bgColor }),
     ...(absolute && {