Pin tailwindcss and check the committed stylesheet against it (closes #231)

make css now runs the standalone tailwindcss CLI, v4.2.1, pinned by
sha256, in a build stage of the Dockerfile instead of whatever
tailwindcss is on the host. A css-check stage, run by make check and
required by the image build, fails when static/css/tailwind.css differs
from what make css generates.

input.css now names its sources (the templates, app.js, and the Go file
that picks status colour classes) instead of letting the whole checkout
be scanned, where words in Go code and docs became unused classes.
.btn-text is removed and tailwind.css regenerated; it only loses unused
rules.

Model: opus-5-5

script/bootstrap

@@ -60,9 +60,9 @@ main() {
     if missing go; then pkg_install go golang go go; fi
 
     # Not installed here: docker is platform-specific and out of scope for a
-    # package-manager bootstrap, but script/lint needs it.
+    # package-manager bootstrap, but script/lint and script/css need it.
     if missing docker; then
-        echo "bootstrap: docker not found; script/lint requires it" >&2
+        echo "bootstrap: docker not found; script/lint and script/css require it" >&2
     fi
 
     go mod download