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/css-check

@@ -0,0 +1,14 @@
+#!/bin/sh
+# script/css-check: fail when static/css/tailwind.css differs from what
+# script/css would generate (read-only). The comparison is the Dockerfile's
+# css-check stage, which the image build runs too.
+set -eu
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+
+main() {
+    cd "$ROOT"
+    docker build --target css-check --output type=cacheonly .
+}
+
+main "$@"