Load Alpine's CSP build so the UI's directives run (closes #371)

The pages' Content-Security-Policy forbids eval, which the standard
Alpine.js build needs, so no directive ran: add forms showed open and
events never collapsed. 3p/ now holds the @alpinejs/csp 3.14.9 tarball
instead, and every directive in templates/ names a property or method
of a component registered in static/js/app.js, as that build requires.
The policy is unchanged.

A browser test in internal/server, built only with the browser tag,
loads the webhook page and the event log under the real headers.
make test-browser runs it in Docker, in a digest-pinned headless
browser image (Dockerfile.browser).

Model: opus-5-5

static/js/app.js

@@ -57,3 +57,73 @@
         init();
     }
 })();
+
+// Alpine.js components.
+//
+// The pages' Content-Security-Policy forbids eval, so the UI loads
+// Alpine's CSP build, which cannot run expressions written in the
+// markup: a directive in templates/ may only name a property or method,
+// and each x-data names a component registered here. This script runs
+// before Alpine, whose script tag is deferred, so this listener is in
+// place when Alpine starts.
+document.addEventListener("alpine:init", function () {
+    "use strict";
+
+    // Something a click shows and hides: the mobile menu, an add form,
+    // an event in the event log, a delivery's attempts.
+    window.Alpine.data("collapsible", function () {
+        return {
+            open: false,
+            toggle() {
+                this.open = !this.open;
+            },
+            get closed() {
+                return !this.open;
+            },
+            // Turns a downward caret up while open.
+            get caretClass() {
+                return { "rotate-180": this.open };
+            },
+        };
+    });
+
+    // The add target form. Only the chosen type's fields show, and the
+    // others are disabled so that the form does not submit them.
+    //
+    // The type is read from the type select when Alpine starts, when the
+    // select changes, and on pageshow. Going back to the page, the
+    // browser restores the type chosen before without a change event,
+    // in some browsers only after Alpine has started, but always before
+    // pageshow.
+    window.Alpine.data("targetForm", function () {
+        return {
+            targetType: "",
+            init() {
+                this.readType();
+            },
+            readType() {
+                this.targetType = this.$root.querySelector(
+                    'select[name="type"]'
+                ).value;
+            },
+            get isHttp() {
+                return this.targetType === "http";
+            },
+            get isSlack() {
+                return this.targetType === "slack";
+            },
+            get isDatabase() {
+                return this.targetType === "database";
+            },
+            get notHttp() {
+                return !this.isHttp;
+            },
+            get notSlack() {
+                return !this.isSlack;
+            },
+            get notDatabase() {
+                return !this.isDatabase;
+            },
+        };
+    });
+});