Commit the Alpine.js tarball in 3p/ and extract it at build time (closes #345)

The build no longer downloads Alpine.js. Its npm package tarball is
committed as 3p/alpinejs-3.14.9.tgz; its sha256 matches the value
script/fetch-assets pinned, and the cdn.min.js inside it matches the
value static/vendor.sha256 pinned.

make assets extracts package/dist/cdn.min.js to the ignored
static/js/alpine.min.js. make test, check, build and dev run it first,
and the Dockerfile builds through make test and make build.
script/fetch-assets, its Dockerfile step, static/vendor.sha256 and
static/vendor_test.go are removed, along with bootstrap's curl install.

Model: opus-5-5

Dockerfile

@@ -51,15 +51,8 @@ RUN go mod download
 # the lint stage above.
 COPY . .
 
-# Fetch the third-party browser assets the UI serves. They are not committed
-# (REPO_POLICIES.md forbids minified bundles in version control) and
-# .dockerignore keeps any host copy out of the build context, so this step is
-# the only way they enter the image. Each download is checked against a
-# hardcoded sha256 and the build fails on mismatch; make test re-checks the
-# hashes against the bytes go:embed actually put in the binary.
-RUN script/fetch-assets
-
-# Run tests and build
+# Run tests and build. Both first run `make assets`, which extracts Alpine.js
+# from its tarball in 3p/.
 RUN make test
 
 # Version stamped into the binary. .dockerignore excludes .git/, so
@@ -67,8 +60,8 @@ RUN make test
 # host and passes it in. The default is what a bare `docker build .`
 # with no --build-arg gets, and it names no tag the tree may not be at.
 #
-# Declared here, below the test and asset steps, so a changed version
-# does not invalidate their cached layers.
+# Declared here, below the test step, so a changed version does not
+# invalidate its cached layer.
 ARG VERSION=unknown
 
 RUN make build VERSION="$VERSION"