feat(terminal): Shift+Enter adds a new line in claude/codex prompts

xterm sends a plain CR for Shift+Enter, so the prompt was submitted. Send
Ctrl+J (LF) instead: both prompts take it as a new line, a shell still runs
the line.

client/src/components/run/TerminalView.tsx

@@ -634,6 +634,15 @@ export const TerminalView = memo(function TerminalView({
           term.input(altArrow, true);
           return false;
         }
+        // Shift+Enter: xterm sends a plain CR, so claude/codex SUBMIT the
+        // prompt. Send Ctrl+J (LF) instead — both prompts take it as a new
+        // line (Option+Enter is the other way, awkward on a Mac), while a
+        // shell still runs the line, as Enter did.
+        if (ev.key === "Enter" && ev.shiftKey && !ev.ctrlKey && !ev.altKey && !ev.metaKey) {
+          ev.preventDefault();
+          term.input("\n", true);
+          return false;
+        }
         if (ev.key === "Tab" && tabMovesRef.current) return false;
         if (ev.key === "Tab" && ev.shiftKey) ev.preventDefault();
         return true;

client/src/components/run/__tests__/TerminalView.test.tsx

@@ -634,6 +634,21 @@ describe("TerminalView", () => {
     ).toBe(true);
   });
 
+  it("sends Shift+Enter as Ctrl+J, a new line in claude's prompt, not a submit", () => {
+    inputMock.mockClear();
+    render(<TerminalView runId="r1" wsBaseUrl="ws://x" />);
+    const handler = keyHandlerMock.mock.calls[0]![0]! as (e: KeyboardEvent) => boolean;
+    const ev = new KeyboardEvent("keydown", { key: "Enter", shiftKey: true, cancelable: true });
+    expect(handler(ev)).toBe(false);
+    expect(ev.defaultPrevented).toBe(true);
+    expect(inputMock).toHaveBeenCalledWith("\n", true);
+    inputMock.mockClear();
+    // Plain Enter and Alt+Enter stay xterm's own.
+    expect(handler(new KeyboardEvent("keydown", { key: "Enter" }))).toBe(true);
+    expect(handler(new KeyboardEvent("keydown", { key: "Enter", altKey: true }))).toBe(true);
+    expect(inputMock).not.toHaveBeenCalled();
+  });
+
   describe("size reporting (tmux must never be wider than the terminal)", () => {
     // A tmux pane wider than xterm wraps each over-long row's tail onto the
     // next row's left margin, and tmux never repaints a row it believes