fix(chat): use teacher conversation placeholder

src/pages/Chat/TeacherChatPanel.tsx

@@ -464,7 +464,7 @@ export function TeacherChatPanel({
         {references.length > 0 && <div className="mb-2 rounded-lg bg-white p-3 text-xs"><p className="line-clamp-3 whitespace-pre-wrap">{references.map((ref) => ref.text).join('\n')}</p><button type="button" className="mt-1 underline" onClick={removeReferences}>移除引用</button></div>}
         <div className="consultation-composer rounded-2xl border bg-white p-3 focus-within:ring-2 focus-within:ring-black/5">
           {activeFocus && <DiscussionFocus label={activeFocus.label} onClear={() => setFocus(null)} />}
-          <textarea ref={inputRef} aria-label={`向${label}提问`} value={text} disabled={busy} maxLength={6000} onChange={(e) => changeText(e.target.value)} placeholder={`想和${label}聊些什么？`} rows={2}
+          <textarea ref={inputRef} aria-label={`向${label}提问`} value={text} disabled={busy} maxLength={6000} onChange={(e) => changeText(e.target.value)} placeholder="和老师聊聊" rows={2}
             onKeyDown={(e) => { if (e.key === 'Enter' && !e.shiftKey && !e.nativeEvent.isComposing) { e.preventDefault(); void send(); } }}
             className="min-h-[60px] w-full resize-none border-0 bg-transparent text-[13px] leading-6 outline-none placeholder:text-muted-foreground/65" />
           <div className="mt-2 flex items-center justify-between gap-2" data-testid="consultation-composer-actions">