The message input supports multi-line text

A single-line input type="text" makes it impossible to paste a multi-line code snippet, error trace, log excerpt, or long-form question — which is precisely the content users want to discuss with an AI. They will either truncate their question, paste it as one giant line that destroys formatting, or abandon the interaction. This is the most common prompt-quality issue in AI chat products and maps directly to lower response quality and lower user satisfaction.

In src/components/chat/ChatInput.tsx, replace the input type="text" with a textarea, auto-resize it based on scrollHeight, and bind Enter to submit while reserving Shift+Enter for newline insertion.

<textarea
  ref={textareaRef}
  rows={1}
  value={input}
  onChange={(e) => { setInput(e.target.value); e.target.style.height = 'auto'; e.target.style.height = e.target.scrollHeight + 'px'; }}
/>