style(chat): restyle message image thumbnails and their expired placeholder

Square the attached-image thumbnail at 100x100 (was 160x120) and give it a
fixed light-gray backdrop so a transparent PNG still shows its artwork —
fixed rather than a neutral token, since the artwork inside the file does
not invert with the theme.

The expired placeholder follows the thumbnail it stands in for: same size,
same tint, no border, lighter icon and label, and the Chinese copy breaks
where it reads best instead of wherever 100px happens to fall.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/frontend/client/src/components/Chat/Messages/Content/InvalidImagePlaceholder.tsx

@@ -18,13 +18,20 @@ export function InvalidImagePlaceholder({
     <div
       {...rest}
       className={cn(
-        'flex h-[120px] w-[160px] flex-col items-center justify-center gap-2 rounded-lg',
-        'border border-border-light bg-surface-secondary text-text-secondary',
+        // pt against a centered stack nudges it down by half the padding — 6px
+        // here, enough to sit off dead-center without looking misaligned.
+        'flex size-[100px] flex-col items-center justify-center gap-2 rounded-lg pt-3',
+        // Same tint as the thumbnail it stands in for, and no border: in a row
+        // of pictures the missing one should hold the place quietly rather than
+        // draw a frame around itself.
+        'bg-[#F8F8F8] text-text-3/60',
         className,
       )}
     >
-      <Outlined.FileImage className="size-7 opacity-60" />
-      <span className="px-2 text-center text-xs leading-tight">
+      <Outlined.FileImage className="size-5" />
+      {/* pre-line so a translation can place its own break; zh splits the two
+          clauses onto their own lines rather than letting 100px decide. */}
+      <span className="whitespace-pre-line px-2 text-center text-caption leading-tight">
         {localize('com_chat_image_expired')}
       </span>
     </div>

src/frontend/client/src/components/Chat/Messages/Content/MessageImage.tsx

@@ -67,7 +67,7 @@ export function MessageImage({
     return (
       <div
         {...debugAttrs}
-        className="h-[120px] w-[160px] animate-pulse rounded-lg bg-fill-2"
+        className="h-[100px] w-[100px] animate-pulse rounded-lg bg-[#F8F8F8]"
       />
     );
   }
@@ -80,7 +80,12 @@ export function MessageImage({
           src={url}
           alt={altText ?? ''}
           onError={() => setFailed(true)}
-          className="h-[120px] w-[160px] cursor-pointer rounded-lg object-cover"
+          // A transparent PNG would otherwise sit on the page's white and lose
+          // its own black artwork; the tint gives it something to show against.
+          // Deliberately a fixed light gray rather than a neutral token: the
+          // artwork inside the file doesn't invert with the theme, so a backdrop
+          // that darkened alongside it would hide the very thing it's here for.
+          className="h-[100px] w-[100px] cursor-pointer rounded-lg bg-[#F8F8F8] object-cover"
         />
       </Dialog.Trigger>
       <DialogImage src={url} />

src/frontend/client/src/locales/zh-Hans/translation.json

@@ -183,7 +183,7 @@
   "com_bschoose_confirm": "确认",
   "com_bschoose_confirmed": "已确认",
   "com_case_featured": "$t(linsight)精选案例",
-  "com_chat_image_expired": "图片已失效，无法查看",
+  "com_chat_image_expired": "图片已失效\n无法查看",
   "com_chat_knowledge_empty_no_org_kbs": "暂无组织知识库",
   "com_chat_knowledge_empty_no_spaces": "暂无知识空间",
   "com_chat_knowledge_placeholder_search_space": "搜索知识空间名称",