feat(run): Enter sends the prompt, and a busy lane attaches instead of failing

- The prompt box sends on Enter (Ctrl/Cmd+Enter still works); Shift+Enter
  inserts a newline, and IME composition (Vietnamese/CJK) never sends.
- Starting or resuming into a lane that still holds a live run answered a
  raw 409 ERUNLIVE; the console now attaches to that run with a notice.
  Checked by error code, since the server's message is prose.
- Active runs gains a two-click Kill for live dashboard-spawned rows, the
  way to free a lane when the previous session really should go.

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

@@ -6,6 +6,7 @@
 
 import { describe, it, expect, vi, beforeEach } from "vitest";
 import { render, screen, fireEvent, waitFor } from "@testing-library/react";
+import { MemoryRouter } from "react-router-dom";
 import { LaneConsolePane } from "../LaneConsolePane";
 import { api } from "../../../lib/api";
 import type { Lane } from "../../../lib/types";
@@ -266,6 +267,64 @@ describe("LaneConsolePane", () => {
     await waitFor(() => expect(prompt.value).toBe("add a button"));
   });
 
+  it("attaches to the lane's still-live session instead of failing when Resume hits ERUNLIVE", async () => {
+    const HISTORY_ITEM = {
+      id: "run-old",
+      session_id: "sess-old",
+      cwd: "/workspace/a",
+      model: null,
+      permission_mode: null,
+      effort: null,
+      resume_session_id: null,
+      prompt_preview: "earlier prompt",
+      status: "abandoned" as const,
+      exit_code: null,
+      started_at: new Date(1000).toISOString(),
+      ended_at: new Date(2000).toISOString(),
+      isLive: false,
+      tool: "claude" as const,
+    };
+    (api.run.history as ReturnType<typeof vi.fn>).mockResolvedValue({ items: [HISTORY_ITEM] });
+    (api.lanes.action as ReturnType<typeof vi.fn>).mockRejectedValue(
+      // Matches the real shape `request()` throws: a plain, human-readable
+      // `.message` plus a `.code` a caller actually branches on.
+      Object.assign(new Error("lane already has a live run"), { code: "ERUNLIVE", status: 409 })
+    );
+    (api.lanes.list as ReturnType<typeof vi.fn>).mockResolvedValue({
+      lanes: [{ ...LANE, run_id: "run-live" }],
+    });
+    (api.run.get as ReturnType<typeof vi.fn>).mockResolvedValue({
+      id: "run-live",
+      laneId: 1,
+      status: "running",
+      cwd: "/workspace/a",
+      model: null,
+      permissionMode: null,
+      effort: null,
+      resumeSessionId: null,
+      sessionId: null,
+      startedAt: null,
+      promptPreview: null,
+    });
+
+    render(
+      <MemoryRouter>
+        <LaneConsolePane {...baseProps()} />
+      </MemoryRouter>
+    );
+
+    fireEvent.click(await screen.findByRole("button", { name: /active runs/i }));
+    const resumeButtons = await screen.findAllByRole("button", {
+      name: "Resume existing session",
+    });
+    fireEvent.click(resumeButtons[0]!);
+
+    await waitFor(() =>
+      expect(screen.getByTestId("terminal-view")).toHaveAttribute("data-run-id", "run-live")
+    );
+    expect(screen.getByText(/attached to it instead/i)).toBeInTheDocument();
+  });
+
   it("renders an empty placeholder with selector when laneId is null but showLaneSelector is true (split-view)", () => {
     render(<LaneConsolePane {...baseProps()} laneId={null} showLaneSelector={true} />);
     expect(screen.getByTestId("pane-empty")).toBeInTheDocument();