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

make css ran whatever tailwindcss binary was on the host's PATH, so the committed stylesheet depended on the machine that built it, and nothing noticed when a template used a class the stylesheet lacked. make css now runs the standalone tailwindcss v4.2.1, pinned by sha256, in a Dockerfile stage, and a check stage, run by make check and required by the image build, fails when the committed static/css/tailwind.css differs from what the templates need, showing the differing rules. input.css names its sources. The unused .btn-text is removed and the stylesheet regenerated, dropping only unused rules. The README has a Stylesheet section.

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;