make test no longer runs the frontend's production build (refs #118)

yarn build moves out of the stage that runs the frontend's unit tests,
now named frontend-test, into a frontend-build stage of its own. It
starts from frontend-test for its yarn packages and source, installs
the git that vite.config.js reads the commit with, and copies from the
lint and test phases as builder does, so it cannot run unless they
passed. The runtime stage copies dist/ from it. The Go compile, most of
make test's time, is unchanged and waits on the ruling asked for in
sneak/prompts#113; no build cache is added.

Model: opus-5-5

README.md

@@ -59,8 +59,9 @@ either one is broken. We provide:
 - `script/clean` — remove what `script/build` writes
 - `script/projectname` — print the project name (used for the Docker image tag)
 - `script/test` — build the `test` phase of `Dockerfile` without the cache: the
-  frontend's unit tests and production build in its `frontend` stage, and the
-  backend's Go tests with the race detector and coverage
+  frontend's unit tests in its `frontend-test` stage, and the backend's Go tests
+  with the race detector and coverage. The frontend's production build is left
+  to the image build, in the `frontend-build` stage
 - `script/lint` — build the `lint` phase of `Dockerfile` without the cache:
   eslint in its `frontend-lint` stage, and golangci-lint over the Go packages
 - `script/fmt` — format all files (writes): prettier over the JavaScript, CSS,
@@ -78,8 +79,8 @@ either one is broken. We provide:
 - `script/frontend-test` — run the unit tests in `test/unit/` on the host with
   Node's built-in test runner, through the `test` script in `package.json`, and
   if any fails, run them again listing every test, and fail; then the production
-  build. Each test run has a 90-second timeout. `make test` runs the same in
-  Docker
+  build. Each test run has a 90-second timeout. `make test` runs the same tests
+  in Docker, without the production build
 - `script/frontend-fmt` — format the JavaScript, CSS, HTML and Markdown with
   prettier (writes)
 - `script/frontend-fmt-check` — check prettier formatting (read-only)