fix: keep line breaks in the fields that are not Markdown

The earlier fix covered the Markdown body and stopped there, so the preview
still ran lines together — which is exactly what it looked like from the
outside: nothing had changed.

Summary, problem, conclusion, environment and the rest are plain text. They
never pass through the Markdown parser, so their newlines sit in a text node
and HTML collapses them, and the renderer that now emits <br> for the body was
never asked about them.

They render through the same rule now. One helper, one behaviour: a line the
author broke stays broken, wherever they typed it.

tests/component/inline-line-breaks.test.tsx

@@ -3,7 +3,10 @@
 import { render } from "@testing-library/react";
 import { describe, expect, it } from "vitest";
 
-import { InlineRenderer } from "../../src/features/tech-log/presentation/shared/public-render/inline-renderer.tsx";
+import {
+  InlineRenderer,
+  PlainText,
+} from "../../src/features/tech-log/presentation/shared/public-render/inline-renderer.tsx";
 
 /**
  * 작성자가 엔터로 나눠 쓴 글이 한 줄로 이어져 보였다. Markdown 이 한 번의 줄바꿈을 문단 내 공백으로
@@ -34,3 +37,19 @@ describe("문단 안의 줄바꿈", () => {
     expect(container.querySelectorAll("br")).toHaveLength(2);
   });
 });
+
+/**
+ * 요약·문제·결론·환경은 Markdown 을 거치지 않고 그대로 그려진다. 본문만 고쳤을 때 이 칸들이
+ * 여전히 이어져 보인 이유이고, 작성자가 "아직 안 고쳐졌다" 고 본 것도 이쪽이다.
+ */
+describe("Markdown 을 거치지 않는 평문 칸", () => {
+  it("작성자가 나눠 쓴 줄을 지킨다", () => {
+    const { container } = render(<PlainText text={"문제 첫 줄\n문제 둘째 줄"} />);
+    expect(container.querySelectorAll("br")).toHaveLength(1);
+  });
+
+  it("한 줄이면 <br> 를 넣지 않는다", () => {
+    const { container } = render(<PlainText text="한 줄" />);
+    expect(container.querySelectorAll("br")).toHaveLength(0);
+  });
+});