fix(stream): preserve filename extension in narrow bubbles

Previously the file bubble wrapped JS middle-ellipsis in CSS truncate,
so narrow containers silently clipped the tail+extension, leaving
e.g. "25cb264a-e06…." instead of "25cb264a-e06…811a.jpg".

Split the displayed name into a shrinking head (with CSS truncate)
and a non-shrinking tail (last 4 base chars + extension). The browser
now decides how much head to clip while the suffix is always visible.

src/components/messageStream/bubbles/FileDocBubble.tsx

@@ -5,10 +5,7 @@ import { useI18n } from "../../../i18n";
 import type { Attachment, Post } from "../../../types/post";
 import { downloadAttachment } from "../utils/downloadFile";
 import { fileIcon } from "../utils/fileIcon";
-import {
-  filenameWithExtension,
-  middleEllipsisFilename,
-} from "../utils/filenameDisplay";
+import { filenameWithExtension, splitFilename } from "../utils/filenameDisplay";
 import { formatBytes } from "../utils/formatBytes";
 import { postDisplayText } from "../utils/postText";
 import { CollapsibleText } from "../CollapsibleText";
@@ -60,10 +57,21 @@ function AttachmentRow({ postId, att }: { postId: string; att: Attachment }) {
       )}
       <div className="min-w-0 flex-1">
         <div
-          className="truncate text-[15px] font-medium leading-6 text-ark-gold group-hover:text-ark-gold2"
+          className="flex min-w-0 items-baseline text-[15px] font-medium leading-6 text-ark-gold group-hover:text-ark-gold2"
           title={displayFilename}
         >
-          {middleEllipsisFilename(displayFilename)}
+          {(() => {
+            const { base, ext } = splitFilename(displayFilename);
+            const tailChars = Math.min(4, base.length);
+            const head = base.slice(0, base.length - tailChars);
+            const tail = base.slice(base.length - tailChars) + ext;
+            return (
+              <>
+                <span className="min-w-0 truncate">{head}</span>
+                <span className="shrink-0 whitespace-pre">{tail}</span>
+              </>
+            );
+          })()}
         </div>
         <div className="text-[12px] font-medium leading-[19.2px] text-[#A8A9AE]">
           {isDownloading ? t("downloading") : formatBytes(att.sizeBytes)}