feat: 消息气泡长文字支持「展开全部/收起全部」折叠

- 新增 CollapsibleText 组件：超过 8 行文字自动折叠，按行高对齐裁切，底部柔和渐隐遮罩暗示有更多内容
- 折叠按钮左对齐，胶囊样式 + chevron 图标，hover 浅背景高亮，点击 chevron 旋转 180°
- 应用到全部 5 种气泡：Text/ImageWithText/Album/Video/FileDoc
- 动画统一使用 motion 包的 EASE_OUT 缓动，尊重 reducedMotion=user
- i18n 七种语言新增 showMore/showLess 文案

src/components/messageStream/bubbles/FileDocBubble.tsx

@@ -11,6 +11,7 @@ import {
 } from "../utils/filenameDisplay";
 import { formatBytes } from "../utils/formatBytes";
 import { postDisplayText } from "../utils/postText";
+import { CollapsibleText } from "../CollapsibleText";
 import { useToast } from "../../Toast";
 
 function AttachmentRow({ postId, att }: { postId: string; att: Attachment }) {
@@ -97,9 +98,9 @@ export function FileDocBubble({ post }: { post: Post }) {
         <AttachmentRow key={att.id} postId={post.id} att={att} />
       ))}
       {text ? (
-        <div className="message-stream-copyable-text select-text whitespace-pre-wrap break-words text-[15px] font-medium leading-6 text-neutral-100">
+        <CollapsibleText className="message-stream-copyable-text select-text whitespace-pre-wrap break-words text-[15px] font-medium leading-6 text-neutral-100">
           {text}
-        </div>
+        </CollapsibleText>
       ) : null}
     </div>
   );