网页版支持粘贴图片发送、聊天图片保存到本地

老板反馈："没有复制粘贴图片功能，只能保存在电脑里面发，图片也没有保存功能"

- 聊天输入框新增粘贴图片检测：剪贴板里有图片（截图工具/复制图片）时直接走发图逻辑，
  不再要求先存到电脑里再用文件选择器上传；普通文本粘贴不受影响
- "查看大图"预览里新增"保存图片"下载按钮，取消息自带的原始文件名，
  取不到时按mime类型生成默认文件名

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

web/src/features/messages/ConversationView.tsx

@@ -936,6 +936,29 @@ export function ConversationView() {
     }
   };
 
+  /** User report (老板, 2026-09-29): "没有复制粘贴图片功能，只能保存在电脑里面发" -- e.g. a screenshot
+   *  tool, or "复制图片" from a browser/chat app, only ever puts raw pixel data on the clipboard, never
+   *  a File the hidden `<input type="file">` above can see; without this, getting that image into a
+   *  message required saving it to disk first and picking it via handlePickImage. `item.getAsFile()`
+   *  returns a real File, indistinguishable from one chosen through the file picker, so this reuses
+   *  the exact same send path. Only preventDefault once an actual image item is found -- an ordinary
+   *  text paste (no image on the clipboard at all) is left completely untouched. */
+  const handleComposerPaste = (e: React.ClipboardEvent<HTMLTextAreaElement>) => {
+    const items = e.clipboardData?.items;
+    if (!items) return;
+    for (let i = 0; i < items.length; i += 1) {
+      const item = items[i];
+      if (item.kind === 'file' && item.type.startsWith('image/')) {
+        const file = item.getAsFile();
+        if (!file) continue;
+        e.preventDefault();
+        const reply = consumePendingReply();
+        sendImage(topic, file, reply).catch((err) => showError(err, '图片发送失败'));
+        return;
+      }
+    }
+  };
+
   return (
     <div className="flex h-full w-full flex-col bg-surface-chat">
       <header className="flex h-15 flex-none items-center justify-between border-b border-border-panel bg-white px-5">
@@ -1214,6 +1237,7 @@ export function ConversationView() {
                   value={draft}
                   onChange={handleDraftChange}
                   onKeyDown={handleKeyDown}
+                  onPaste={handleComposerPaste}
                   placeholder="输入消息，Enter 发送，Shift+Enter 换行"
                   className="min-h-[72px] flex-1 resize-none bg-transparent text-[15px] leading-normal outline-none placeholder:text-text-placeholder-1"
                 />