build: pin one Node major across the dev, CI and build machines

The server gate ran node:22 while the dev gate ran node:24, and the two disagree about
Intl.NumberFormat's compact currency padding and about which realm's Blob a Response hands
back — so the frontend job had been red for 73 of its last 90 runs while the local gate was
green, and nothing said so.

Align everything on 24, with `.nvmrc` as the single source of truth: the CI container, the
prod build stage, `engines`, and `@types/node` (which was two majors ahead of the runtime, so
tsc was green over APIs the shipped engine does not have). The backend job's node only runs
the checkout action, so it tracks the pin as hygiene — node 20 is EOL — but deliberately
stays outside the contract.

The pin is held by a contract test that reads every one of those places AND the engine
running it, so the next drift is a red lane on its first run instead of a CI log nobody reads.

Verified on node:24-bookworm (the CI lane: 228 files / 1552 tests, typecheck, lint,
format:check, knip all exit 0) and on node:24-alpine (the prod build stage, v24.21.0, SPA
built). Each assertion of the new test was mutation-checked red.

.gitea/workflows/check.yml

@@ -7,8 +7,12 @@
 # runs pytest NATIVELY against a `postgres` service (not the local docker-compose.test.yml, whose
 # bind-mount is unreliable under a mounted-socket runner); fidelity is kept by installing the SAME pinned
 # requirements, Postgres 16, and ffmpeg, and by setting SIFTLODE_TEST_REQUIRE_DB=1 so a DB/ffmpeg that
-# failed to come up is a hard error, never a silent skip. node 22 matches the prod build image
-# (Dockerfile `node:22-alpine`).
+# failed to come up is a hard error, never a silent skip.
+#
+# The frontend container's node major is NOT free: it must equal the prod build image's and the dev
+# machine's, because vitest/tsc read the engine's own Intl and web-API implementations and those split
+# between majors. `.nvmrc` is the single source of truth and
+# `frontend/src/lib/nodeEngine.contract.test.ts` fails this gate if any of them drift apart.
 name: gate
 
 on: [push, pull_request]
@@ -16,7 +20,7 @@ on: [push, pull_request]
 jobs:
   frontend:
     runs-on: ubuntu-latest
-    container: node:22-bookworm
+    container: node:24-bookworm
     defaults:
       run:
         working-directory: frontend
@@ -34,7 +38,11 @@ jobs:
     # A combined python+node image: python 3.12 (matches the prod Dockerfile) for the tests, and node
     # so the JS actions (checkout) run. actions/setup-python is NOT usable here — the runner can't reach
     # GitHub's python-versions download, so it must be baked into the image.
-    container: nikolaik/python-nodejs:python3.12-nodejs20
+    #
+    # This node runs the checkout action, never our code, so it is deliberately OUTSIDE the engine
+    # contract the frontend job is held to — it tracks `.nvmrc` as hygiene (node 20 went EOL in April
+    # 2026), but a missing upstream tag here must never be able to block the gate.
+    container: nikolaik/python-nodejs:python3.12-nodejs24
     services:
       postgres:
         image: postgres:16-alpine