refactor(platform): extract blurActiveElement to infrastructure module

- Centralize duplicated Platform.OS === 'web' blur logic across 16 components
- Add blurActiveElement utility to src/infrastructure/platform module
- Optimize MessageBubble and ImageSegment with React.memo custom comparators
- Add useMemo for computed values in MessageSegmentsRenderer
- Update DTO types with is_system_notice and notice_content fields
- Fix keyboard dismissal order in useChatScreen handleDismiss
- Simplify userStore follow/unfollow state updates
- Remove unnecessary TypeScript type assertions throughout codebase

src/infrastructure/platform/domUtils.ts

@@ -0,0 +1,83 @@
+/**
+ * Web Platform DOM Utilities
+ * 
+ * 提供Web平台特定的DOM操作工具函数
+ * 解决跨平台代码中需要访问Web DOM的补丁问题
+ */
+
+import { Platform, Keyboard } from 'react-native';
+
+/**
+ * Blur当前激活的DOM元素（Web）并收起键盘（移动端）
+ * 
+ * - Web: blur DOM active element
+ * - iOS/Android: Keyboard.dismiss()
+ * 
+ * 替代散落在多个文件中的重复代码
+ */
+export function blurActiveElement(): void {
+  if (Platform.OS === 'web') {
+    try {
+      const activeElement = (globalThis as any)?.document?.activeElement as 
+        { blur?: () => void } | undefined;
+      
+      if (activeElement?.blur) {
+        activeElement.blur();
+      }
+    } catch {
+      // 安全忽略：某些环境下document可能不可访问
+    }
+  } else {
+    Keyboard.dismiss();
+  }
+}
+
+/**
+ * 检查当前是否有激活的DOM元素（仅Web平台）
+ */
+export function hasActiveElement(): boolean {
+  if (Platform.OS !== 'web') return false;
+  
+  try {
+    const doc = (globalThis as any)?.document;
+    return !!doc?.activeElement && doc.activeElement !== doc.body;
+  } catch {
+    return false;
+  }
+}
+
+/**
+ * 获取当前激活元素的类型（仅Web平台）
+ * 用于判断是否需要特殊处理（如输入框）
+ */
+export function getActiveElementType(): string | null {
+  if (Platform.OS !== 'web') return null;
+  
+  try {
+    const activeElement = (globalThis as any)?.document?.activeElement as 
+      { tagName?: string; type?: string } | undefined;
+    
+    if (!activeElement) return null;
+    
+    const tagName = activeElement.tagName?.toLowerCase();
+    const type = activeElement.type?.toLowerCase();
+    
+    return tagName === 'input' ? `input:${type || 'text'}` : tagName || null;
+  } catch {
+    return null;
+  }
+}
+
+/**
+ * 判断当前激活元素是否为输入类型
+ */
+export function isInputFocused(): boolean {
+  const elementType = getActiveElementType();
+  if (!elementType) return false;
+  
+  return (
+    elementType.startsWith('input:') ||
+    elementType === 'textarea' ||
+    elementType === 'select'
+  );
+}

src/infrastructure/platform/index.ts

@@ -0,0 +1,12 @@
+/**
+ * Platform Infrastructure
+ * 
+ * 平台相关的基础设施代码
+ */
+
+export {
+  blurActiveElement,
+  hasActiveElement,
+  getActiveElementType,
+  isInputFocused,
+} from './domUtils';