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 headless Chromium test in internal/server loads the webhook page and
the event log under the real headers. The Dockerfile's test stage
installs chromium; where it is missing the test skips.

Model: opus-5-5

README.md

@@ -1270,9 +1270,20 @@ We provide:
 
 ## Third-party browser assets
 
-The web UI serves one third-party script, Alpine.js. Its npm package tarball
-is committed as `3p/alpinejs-3.14.9.tgz`, byte for byte as the npm registry
-publishes it. It is a dependency, not this repo's build output, so
+The web UI serves one third-party script, Alpine.js, in its CSP build: the npm
+package `@alpinejs/csp`. The pages' Content-Security-Policy forbids eval, which
+the standard `alpinejs` build needs to run the expressions written in the
+markup. The CSP build runs no expressions, so every Alpine directive in
+`templates/` only names a property or method of a component registered in
+`static/js/app.js`: `x-data="collapsible"` and `@click="toggle"`, never
+`x-data="{ open: false }"` or `@click="open = !open"`. A browser test in
+`internal/server` loads the pages under the real policy and fails on any
+directive that does not work. It needs `chromium` on `PATH` and skips without
+it; the Dockerfile installs it, so the image build always runs it.
+
+The package's tarball is committed as `3p/alpinejs-csp-3.14.9.tgz`, byte for
+byte as the npm registry publishes it. It is a dependency, not this repo's
+build output, so
 `REPO_POLICIES.md`'s rule against committed build artifacts does not apply.
 The directory is `3p/` rather than `vendor/` because Go treats a root
 `vendor/` directory as its module vendor directory.
@@ -1285,10 +1296,11 @@ nothing downloads Alpine.js. The extracted file is not committed, and
 `.dockerignore` keeps any host copy out of the build context.
 
 To move to a new version: download
-`https://registry.npmjs.org/alpinejs/-/alpinejs-<version>.tgz`, check it
+`https://registry.npmjs.org/@alpinejs/csp/-/csp-<version>.tgz`, check it
 against the `dist.integrity` hash listed at
-`https://registry.npmjs.org/alpinejs/<version>`, replace the tarball in `3p/`
-with it, update its file name in `script/assets`, and run `make check`.
+`https://registry.npmjs.org/@alpinejs/csp/<version>`, replace the tarball in
+`3p/` with it as `alpinejs-csp-<version>.tgz`, update its file name in
+`script/assets`, and run `make check`.
 
 ## Rationale
 
@@ -2769,7 +2781,7 @@ imports. The entry point is `cmd/webhooker/main.go`.
 ```
 webhooker/
 ├── 3p/
-│   └── alpinejs-3.14.9.tgz    # Alpine.js npm package, extracted by make assets
+│   └── alpinejs-csp-3.14.9.tgz  # Alpine.js CSP build npm package, extracted by make assets
 ├── cmd/webhooker/
 │   └── main.go               # Entry point: subcommand dispatch; no args locks DATA_DIR and wires fx
 ├── internal/
@@ -2862,8 +2874,8 @@ webhooker/
 │   ├── css/input.css          # Tailwind input, source for tailwind.css (make css)
 │   ├── css/tailwind.css       # Generated stylesheet the pages load
 │   ├── css/style.css          # Older hand-written stylesheet, no longer loaded
-│   ├── js/app.js              # Progressive-enhancement copy-to-clipboard
-│   └── js/alpine.min.js       # Alpine.js, extracted from 3p/ by make assets, not committed
+│   ├── js/app.js              # Copy-to-clipboard, and the Alpine.js components
+│   └── js/alpine.min.js       # Alpine.js CSP build, extracted from 3p/ by make assets, not committed
 ├── templates/                 # Go HTML templates (base, login, sources, etc.)
 ├── script/                    # Scripts to Rule Them All entrypoints
 ├── Dockerfile                 # Three stages: lint, test+build, Alpine runtime