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/ImageWithTextBubble.tsx

@@ -5,6 +5,7 @@ import { AttachmentDownloadPill } from "../AttachmentDownloadPill";
 import { BubbleImage } from "../BubbleImage";
 import { autolink } from "../utils/autolink";
 import { postDisplayText } from "../utils/postText";
+import { CollapsibleText } from "../CollapsibleText";
 
 export function ImageWithTextBubble({ post }: { post: Post }) {
   const { openLightbox } = useLightbox();
@@ -30,9 +31,12 @@ export function ImageWithTextBubble({ post }: { post: Post }) {
         <AttachmentDownloadPill postId={post.id} attachment={att} />
       </div>
       {text ? (
-        <div className="message-stream-copyable-text select-text whitespace-pre-wrap break-words px-4 pt-3 text-[14px] leading-6 text-neutral-100">
+        <CollapsibleText
+          wrapperClassName="px-4 pt-3"
+          className="message-stream-copyable-text select-text whitespace-pre-wrap break-words text-[14px] leading-6 text-neutral-100"
+        >
           {autolink(text)}
-        </div>
+        </CollapsibleText>
       ) : null}
     </div>
   );