feat(AI设计): 支持输入框回车发送

需求：AI 设计输入框监听 Enter 并发送消息。

实现：复用现有发送链路，保留 Shift+Enter 换行并保护输入法组合状态；补充聚焦回归测试。

src/pages/ImageCanvas/index.tsx

@@ -536,10 +536,9 @@ export function ImageCanvas() {
   };
 
   const handleComposerKeyDown = (event: KeyboardEvent<HTMLTextAreaElement>) => {
-    if ((event.metaKey || event.ctrlKey) && event.key === 'Enter') {
-      event.preventDefault();
-      void handleSend();
-    }
+    if (event.key !== 'Enter' || event.shiftKey || event.nativeEvent.isComposing) return;
+    event.preventDefault();
+    void handleSend();
   };
 
   const handleFirstFrameSelect = async (asset: DesignAsset) => {
@@ -819,7 +818,7 @@ export function ImageCanvas() {
                 />
                 {actionError ? <p role="alert" className="mt-2 rounded-xl bg-destructive/10 px-3 py-2 text-xs font-semibold text-destructive">{actionError}</p> : null}
                 <div className="mt-2 flex items-center justify-between gap-3 border-t border-border/70 pt-2">
-                  <span className="text-[10px] font-medium text-muted-foreground">⌘/Ctrl + Enter 发送</span>
+                  <span className="text-[10px] font-medium text-muted-foreground">Enter 发送 · Shift+Enter 换行</span>
                   <Button type="submit" className="h-9 rounded-full bg-brand px-4 text-primary-foreground" disabled={submitting || !prompt.trim()}>
                     {submitting ? <Loader2 className="mr-2 h-4 w-4 animate-spin" /> : <Send className="mr-2 h-4 w-4" />}
                     发送给设计 Agent