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

make css now runs the Tailwind v4.2.1 standalone CLI in a docker build
stage, the binary pinned by sha256, 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 Tailwind scan the
whole checkout, 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/check

@@ -1,8 +1,8 @@
 #!/bin/sh
-# script/check: run all checks (test, lint, fmt-check). Our own
+# script/check: run all checks (test, lint, fmt-check, css-check). Our own
 # extension to scripts-to-rule-them-all.
 # Writes only the ignored static/js/alpine.min.js, through script/test.
-# Generic: usually needs no adaptation.
+# Generic, apart from css-check.
 set -eu
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
@@ -11,6 +11,7 @@ main() {
     "$SCRIPT_DIR/test"
     "$SCRIPT_DIR/lint"
     "$SCRIPT_DIR/fmt-check"
+    "$SCRIPT_DIR/css-check"
 }
 
 main "$@"