fix: keep the welcome composer at the bottom

tests/e2e/fixtures/community-welcome.tsx

@@ -35,7 +35,7 @@ export function Layout() {
     <main className="classroom-main flex min-h-0 min-w-0 flex-1 flex-col">
       <div className="classroom-tabs" role="tablist"><button role="tab" aria-selected="true">操作对话</button><button role="tab">作品</button></div>
       <CodingChatBody welcome={!sent} hidden={false} bubble={bubble}>
-        <div className="classroom-chat-view min-h-0 flex-1">{!sent ? <CodingStartHeading /> : <p>第一条消息已发送（测试）</p>}</div>
+        <div className="classroom-chat-view min-h-0 flex-1 flex-col">{!sent ? <><CodingStartHeading /><CommunityInvitation /></> : <p>第一条消息已发送（测试）</p>}</div>
         <div className="classroom-composer-view shrink-0">
           <CodingComposer value={draft} editable canSend={Boolean(draft)} preparing={false} recovering={false}
             runStatus="idle" mode="prompt" queue={{ items: [] }} error={null} recoverableError={false}
@@ -43,7 +43,6 @@ export function Layout() {
             onModeChange={() => {}} onSubmit={() => { setSent(true); setDraft(''); }} onRecover={() => {}}
             onAddFiles={() => {}} onRemoveAttachment={() => {}} />
         </div>
-        {!sent && <CommunityInvitation />}
       </CodingChatBody>
     </main>
   </div>;