fix(build): keep JSX line-end spaces in dev and vitest

plugin-react recognises the React Compiler only by its package name, and
reactPlugin.ts passes the imported plugin, so plugin-react left Babel's
retainLines on in dev. With the compiler's re-printed JSX that trimmed the
space at the end of a text line (the privacy page read
"for example/api/feed" on :5173). Production builds never had it on.
Turn retainLines off through generatorOpts, which wins over the plugin's
top-level option, and pin it with a DOM test seen red without the fix.

frontend/reactPlugin.ts

@@ -18,9 +18,17 @@ import reactCompiler from "babel-plugin-react-compiler";
 // reasons, both about the gate: a string is invisible to knip, which then reports the dependency as
 // unused, and — worse — a typo in that string is a silent no-op that ships an uncompiled bundle,
 // while a bad import is a resolve error on the tsc lane.
+//
+// The price of the import: plugin-react finds the compiler only by that string, so it doesn't know the
+// compiler is there. Its one compiler-specific switch is Babel's `retainLines`, which it turns on for
+// dev (and so for vitest) unless the compiler is present. With the compiler's re-printed JSX that eats
+// the space at the end of a text line ("for example <code>" rendered "for example/api/feed"). The
+// production build never had it on. `generatorOpts` wins over the top-level option, so it is switched
+// off here, for every mode. Pinned by src/jsxWhitespace.dom.test.tsx.
 export const reactWithCompiler = () =>
   react({
     babel: {
       plugins: [[reactCompiler, { target: "19" }]],
+      generatorOpts: { retainLines: false },
     },
   });

frontend/src/jsxWhitespace.dom.test.tsx

@@ -0,0 +1,35 @@
+// @vitest-environment jsdom
+//
+// The React transform must not eat the space between a word and the element or expression that
+// follows it on the same line. It did, in dev and in this test lane (never in the production build):
+// @vitejs/plugin-react only recognises the React Compiler by its package NAME, and reactPlugin.ts
+// passes the imported plugin, so the React plugin kept Babel's `retainLines` on. The compiler then
+// printed "for example " and the next element on different lines, and the JSX transform trimmed the
+// line-end space ("for example/api/feed" on the privacy page). reactPlugin.ts now turns
+// `retainLines` off itself. The component goes through the same transform the app does.
+import { describe, it, expect, afterEach } from "vitest";
+import { render, cleanup } from "@testing-library/react";
+
+afterEach(cleanup);
+
+function Sentence({ value }: { value: string }) {
+  // The line breaks ARE the test: collapsed onto one line, the defect doesn't fire.
+  // prettier-ignore
+  return (
+    <p>
+      which part of the app was called (for example <code>/api/feed</code>, never the search
+      <span>
+        status: {value} end
+      </span>
+    </p>
+  );
+}
+
+describe("JSX text whitespace through the React transform", () => {
+  it("keeps the space before an element and before an expression", () => {
+    const { container } = render(<Sentence value="ok" />);
+    expect(container.textContent).toBe(
+      "which part of the app was called (for example /api/feed, never the searchstatus: ok end",
+    );
+  });
+});