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, byte for byte the file script/fetch-assets downloaded, with the sha256 that script pinned. script/assets (make assets) extracts package/dist/cdn.min.js to the ignored static/js/alpine.min.js; script/test runs it, so make test, make check, the pre-commit hook and the Dockerfile get the file with no network access, and make build, run and dev run it too.

Removed: script/fetch-assets, its Dockerfile step, static/vendor.sha256 and static/vendor_test.go. The README describes the new flow.

Model: opus-5-5

Makefile

@@ -28,7 +28,7 @@ setup:
 	@script/setup
 
 assets:
-	@script/fetch-assets
+	@script/assets
 
 test:
 	@script/test
@@ -45,13 +45,13 @@ fmt-check:
 check:
 	@script/check
 
-build:
+build: assets
 	go build -ldflags '$(strip -X main.version=$(VERSION) $(GO_LDFLAGS))' -o bin/webhooker ./cmd/webhooker
 
 run: build
 	./bin/webhooker
 
-dev:
+dev: assets
 	go run ./cmd/webhooker
 
 deps: