feat: Shift+Enter inserts newline in chat input (story 82)

Change chat input from <input> to <textarea> so multi-line messages
are supported. Enter (without Shift) still sends; Shift+Enter inserts
a newline via the native textarea behavior.

- Replace <input> with <textarea rows={1}> in Chat.tsx
- Update onKeyDown: guard sendMessage() with !e.shiftKey check
  and call e.preventDefault() to suppress the default newline on Enter
- Update inputRef type from HTMLInputElement to HTMLTextAreaElement
- Add style props: resize:none, overflowY:auto, fontFamily:inherit
- Add 3 new Vitest tests covering all 3 acceptance criteria (AC1-AC3)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/Chat.tsx

@@ -44,7 +44,7 @@ export function Chat({ projectPath, onCloseProject }: ChatProps) {
 
 	const wsRef = useRef<ChatWebSocket | null>(null);
 	const messagesEndRef = useRef<HTMLDivElement>(null);
-	const inputRef = useRef<HTMLInputElement>(null);
+	const inputRef = useRef<HTMLTextAreaElement>(null);
 	const scrollContainerRef = useRef<HTMLDivElement>(null);
 	const shouldAutoScrollRef = useRef(true);
 	const lastScrollTopRef = useRef(0);
@@ -652,16 +652,18 @@ export function Chat({ projectPath, onCloseProject }: ChatProps) {
 								alignItems: "center",
 							}}
 						>
-							<input
+							<textarea
 								ref={inputRef}
 								value={input}
 								onChange={(e) => setInput(e.target.value)}
 								onKeyDown={(e) => {
-									if (e.key === "Enter") {
+									if (e.key === "Enter" && !e.shiftKey) {
+										e.preventDefault();
 										sendMessage();
 									}
 								}}
 								placeholder="Send a message..."
+								rows={1}
 								style={{
 									flex: 1,
 									padding: "14px 20px",
@@ -673,6 +675,9 @@ export function Chat({ projectPath, onCloseProject }: ChatProps) {
 									background: "#2f2f2f",
 									color: "#ececec",
 									boxShadow: "0 2px 6px rgba(0,0,0,0.02)",
+									resize: "none",
+									overflowY: "auto",
+									fontFamily: "inherit",
 								}}
 							/>
 							<button