Drop 'unsafe-inline' from the Content-Security-Policy (closes #530)

The delete prompts on the webhook page were inline onsubmit handlers.
Each delete form now carries its question in a data-confirm attribute,
which one submit listener in static/js/app.js asks. The three style
attributes became Tailwind classes, with tailwind.css regenerated, and
the x-cloak rule moved to style.css. The policy now sends
script-src 'self' and style-src 'self'.

The browser test now answers a target's delete prompt no and then yes,
and also loads the sign-in page, the webhook list, the settings and
profile pages and the 404 error page under the policy. The README
states the policy as sent.

Model: opus-5-5

static/css/style.css

@@ -1,7 +1,7 @@
 /*
- * The two shared styles for the controls a user clicks. Every page loads
- * this file after tailwind.css. It is plain CSS: make css does not build
- * it.
+ * The two shared styles for the controls a user clicks, and the rule that
+ * hides an element marked x-cloak. Every page loads this file after
+ * tailwind.css. It is plain CSS: make css does not build it.
  *
  * A button is btn-primary, btn-secondary or btn-danger, from input.css.
  * A secondary or inline action, such as Copy beside an entrypoint URL or
@@ -46,3 +46,11 @@
         outline-offset: 2px;
     }
 }
+
+/*
+ * Alpine.js removes x-cloak from an element once it has started, so
+ * something Alpine hides is not seen on the page while it loads.
+ */
+[x-cloak] {
+    display: none !important;
+}

static/js/app.js

@@ -58,6 +58,18 @@
     }
 })();
 
+// Asks before a form with a data-confirm attribute is sent, such as a
+// delete form on the webhook page. The browser's own prompt shows the
+// attribute's text, and the form is sent only if the user accepts.
+document.addEventListener("submit", function (event) {
+    "use strict";
+
+    const message = event.target.getAttribute("data-confirm");
+    if (message !== null && !window.confirm(message)) {
+        event.preventDefault();
+    }
+});
+
 // Alpine.js components.
 //
 // The pages' Content-Security-Policy forbids eval, so the UI loads