feat(HAI-013): add markdown rendering styles and TaskDetailModal tests

- Add markdown-body base styles in dashboard styles.css
- Remove backdrop and enable scroll on task detail prompt rendering
- Add TaskDetailModal unit tests with vitest jsdom environment
- Update vitest config with globals and jsdom support
- Clean up unused test files and legacy concurrency module

packages/dashboard/app/components/TaskDetailModal.tsx

@@ -117,7 +117,7 @@ export function TaskDetailModal({
           <div className="detail-section">
             <h4>PROMPT.md</h4>
             {task.prompt ? (
-              <div className="detail-prompt markdown-body">
+              <div className="markdown-body">
                 <ReactMarkdown remarkPlugins={[remarkGfm]}>
                   {task.prompt}
                 </ReactMarkdown>

packages/dashboard/app/components/__tests__/TaskDetailModal.test.tsx

@@ -0,0 +1,77 @@
+import { describe, it, expect, vi } from "vitest";
+import { render, screen } from "@testing-library/react";
+import { TaskDetailModal } from "../TaskDetailModal";
+import type { TaskDetail, Column, MergeResult, Task } from "@hai/core";
+
+function makeTask(overrides: Partial<TaskDetail> = {}): TaskDetail {
+  return {
+    id: "HAI-099",
+    description: "Test task",
+    column: "in-progress" as Column,
+    dependencies: [],
+    steps: [],
+    currentStep: 0,
+    log: [],
+    createdAt: "2026-01-01T00:00:00Z",
+    updatedAt: "2026-01-01T00:00:00Z",
+    ...overrides,
+  };
+}
+
+const noop = vi.fn();
+const noopMove = vi.fn(async () => ({}) as Task);
+const noopDelete = vi.fn(async () => ({}) as Task);
+const noopMerge = vi.fn(async () => ({ merged: false }) as MergeResult);
+
+describe("TaskDetailModal", () => {
+  it("renders markdown-body without detail-prompt class when prompt exists", () => {
+    const { container } = render(
+      <TaskDetailModal
+        task={makeTask({ prompt: "# Hello\n\nSome **bold** text" })}
+        onClose={noop}
+        onMoveTask={noopMove}
+        onDeleteTask={noopDelete}
+        onMergeTask={noopMerge}
+        addToast={noop}
+      />,
+    );
+
+    const markdownDiv = container.querySelector(".markdown-body");
+    expect(markdownDiv).toBeTruthy();
+    expect(markdownDiv!.classList.contains("detail-prompt")).toBe(false);
+  });
+
+  it("renders ReactMarkdown output (heading and bold text)", () => {
+    const { container } = render(
+      <TaskDetailModal
+        task={makeTask({ prompt: "# Hello\n\nSome **bold** text" })}
+        onClose={noop}
+        onMoveTask={noopMove}
+        onDeleteTask={noopDelete}
+        onMergeTask={noopMerge}
+        addToast={noop}
+      />,
+    );
+
+    expect(container.querySelector("h1")?.textContent).toBe("Hello");
+    expect(container.querySelector("strong")?.textContent).toBe("bold");
+  });
+
+  it("renders (no prompt) with detail-prompt class when prompt is absent", () => {
+    const { container } = render(
+      <TaskDetailModal
+        task={makeTask({ prompt: undefined })}
+        onClose={noop}
+        onMoveTask={noopMove}
+        onDeleteTask={noopDelete}
+        onMergeTask={noopMerge}
+        addToast={noop}
+      />,
+    );
+
+    const fallback = screen.getByText("(no prompt)");
+    expect(fallback).toBeTruthy();
+    expect(fallback.classList.contains("detail-prompt")).toBe(true);
+    expect(fallback.classList.contains("markdown-body")).toBe(false);
+  });
+});