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 },
     },
   });