build(frontend): finish the React 19 move, and hold the versions that were left in prose

react-i18next was still on ^14.1.2. The E27 S1 plan named it as "the one peer that
actually needs moving — React 19 support is the 15 line", and then it did not
move. The 14 line's <Trans> reads `element.ref`, removed in React 19, and its
withTranslation types are built on the GLOBAL JSX namespace @types/react 19 no
longer declares — both latent only because this app imports neither. 15.7.4 fixes
both at the source (`c.props.ref ?? c.ref`, `React.JSX.LibraryManagedAttributes`).

`reactEngine.contract.test.ts` joins its Node and Python twins and holds what was
only a comment: the compiler's `target` against the installed React major, react
and react-dom together, the types against the runtime, and react-i18next's floor.
A stale target is not a corrupt bundle — the plugin validates the option — but it
is a silent one, since tsc never sees Babel's output.

pythonEngine gains the seventh spelling nobody held: `requirements.lock`'s own
regeneration recipe names a python image, and wheels resolve against THAT
interpreter, so a recipe left behind yields a lock that does not match the image
installing it.

The type lane now covers `alembic` too, for the reason the ruff lane was widened
to it — migrations run on every container start, so one that raises crash-loops
prod. It came back at zero new errors, so the baseline is untouched. `tests/` is
deliberately still out, and the config says why.

vitest.config's docblock claimed the React transform is "a no-op for the plain .ts
node tests". It is not: the compiler rewrites src/lib's hooks in that lane too,
which is the point of sharing one factory with the build — and worth knowing
before ruling the compiler out of a divergence.

Tests: useCardPager had none, in either the hook or its two callers, though its
page clamp moved to render in S2 — the transformation where a non-convergent
condition is a crash rather than a glitch. Four cases now, including termination
on an empty list. Combobox's and Avatar's close/failed-URL cases are relabelled
CHARACTERIZATION: both pass against the pre-sweep code, and the tests beside them
are the ones that discriminate.

frontend/vitest.config.ts

@@ -6,7 +6,11 @@ import { reactWithCompiler } from "./reactPlugin";
 // DOM). A test that needs the DOM (component render or renderHook) opts INTO jsdom per-file with a
 //   // @vitest-environment jsdom
 // docblock on its first line. plugin-react is wired here so those tests get JSX + a Fast-Refresh-
-// free React transform; it is a no-op for the plain .ts node tests (they carry no JSX). jsdom is a
+// free React transform. It is NOT a no-op for the plain .ts node tests: the plugin's filter is
+// /\.[tj]sx?$/, so the React Compiler rewrites `src/lib`'s hooks in that lane too (usePref,
+// useCardPager, plexPlayerHooks all come out with `react/compiler-runtime` memo slots). That is the
+// point of sharing one factory with vite.config.ts — the gate compiles the code the way the build
+// does — and it is worth knowing before ruling the compiler out of a divergence. jsdom is a
 // devDependency referenced only via that docblock env string — knip's vitest plugin still sees it.
 export default defineConfig({
   plugins: [reactWithCompiler()],