feat(ui): redesign system message item with card-style layout and add title validation

- Refactor SystemMessageItem with modern card design, gradient colors, and improved visual hierarchy
- Add unread indicator, status badges, and enhanced avatar badges for message types
- Update CreatePostScreen to require title input and improve placeholder text
- Add Android back button handling for notification screens in embedded mode
- Replace manual shadow styles with theme shadow utilities across message screens

src/screens/create/CreatePostScreen.tsx

@@ -337,6 +337,11 @@ export const CreatePostScreen: React.FC<CreatePostScreenProps> = (props) => {
     // 防止重复点击
     if (posting) return;
 
+    if (!title.trim()) {
+      Alert.alert('错误', '请输入帖子标题');
+      return;
+    }
+
     if (!content.trim()) {
       Alert.alert('错误', '请输入帖子内容');
       return;
@@ -365,7 +370,7 @@ export const CreatePostScreen: React.FC<CreatePostScreenProps> = (props) => {
         
         // 创建投票帖子
         await voteService.createVotePost({
-          title: title.trim() || '无标题',
+          title: title.trim(),
           content: content.trim(),
           images: imageUrls,
           vote_options: validOptions,
@@ -383,7 +388,7 @@ export const CreatePostScreen: React.FC<CreatePostScreenProps> = (props) => {
       } else {
         if (isEditMode && editPostID) {
           const updated = await postService.updatePost(editPostID, {
-            title: title.trim() || '无标题',
+            title: title.trim(),
             content: content.trim(),
             images: imageUrls,
           });
@@ -403,7 +408,7 @@ export const CreatePostScreen: React.FC<CreatePostScreenProps> = (props) => {
         } else {
           // 创建普通帖子
           await postService.createPost({
-            title: title.trim() || '无标题',
+            title: title.trim(),
             content: content.trim(),
             images: imageUrls,
           });
@@ -562,7 +567,7 @@ export const CreatePostScreen: React.FC<CreatePostScreenProps> = (props) => {
         ]}
         value={title}
         onChangeText={setTitle}
-        placeholder="添加标题（可选）"
+        placeholder="请输入标题（必填）"
         placeholderTextColor={colors.text.hint}
         maxLength={MAX_TITLE_LENGTH}
       />