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

static/css/input.css

@@ -1,7 +1,10 @@
-@import "tailwindcss";
-
-/* Source the templates */
+/* Classes are taken only from the files named below. A class written in
+   any other file is not generated: name that file here too. */
+@import "tailwindcss" source(none);
 @source "../../templates/**/*.html";
+@source "../js/app.js";
+/* targetStatus picks a target's status colour class */
+@source "../../internal/handlers/recent_events.go";
 
 /* Material Design inspired theme customization */
 @theme {
@@ -53,10 +56,6 @@
     @apply inline-flex items-center justify-center px-4 py-2 rounded-md font-medium text-sm transition-all duration-200 focus:outline-none focus:ring-2 focus:ring-offset-2 disabled:opacity-50 disabled:cursor-not-allowed bg-error-500 text-white hover:bg-error-700 active:bg-red-800 focus:ring-red-500 shadow-elevation-1 hover:shadow-elevation-2;
   }
 
-  .btn-text {
-    @apply inline-flex items-center justify-center px-4 py-2 rounded-md font-medium text-sm transition-all duration-200 focus:outline-none focus:ring-2 focus:ring-offset-2 disabled:opacity-50 disabled:cursor-not-allowed text-primary-600 hover:bg-primary-50 active:bg-primary-100;
-  }
-
   /* Cards */
   .card {
     @apply bg-white rounded-lg shadow-elevation-1 overflow-hidden;