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

README.md

@@ -19,8 +19,8 @@ before deploying one.
 ### Prerequisites
 
 - Go 1.26.1+ (the version in `go.mod`)
-- Docker (for linting, for the test stage of the CI gate, and for
-  containerized deployment)
+- Docker (for linting, for the browser test, for the test stage of the
+  CI gate, and for containerized deployment)
 
 golangci-lint is not a prerequisite and must not be installed on the
 host: `script/bootstrap` does not install it, and `make lint` runs the
@@ -58,6 +58,7 @@ make fmt        # Format code (gofmt + goimports)
 make fmt-check  # Fail if gofmt would change anything (writes nothing)
 make lint       # Run golangci-lint in Docker (Dockerfile.lint)
 make test       # Run tests with race detection
+make test-browser # Run the browser test in Docker (Dockerfile.browser)
 make check      # test + lint + fmt-check (CI gate)
 make build      # Build binary to bin/webhooker (version-stamped)
 make version    # Print the version this checkout would stamp
@@ -1224,7 +1225,7 @@ What that means for an operator:
 This repository adheres to the
 [Scripts to Rule Them All](https://github.com/github/scripts-to-rule-them-all)
 standard: normalized scripts in `script/` are the entrypoints for the
-development workflow. Ten of the Makefile's seventeen targets are thin
+development workflow. Eleven of the Makefile's eighteen targets are thin
 shims that call them; `build`, `run`, `dev`, `deps`, `clean`, `css` and
 `version` are inline commands with no script behind them, though `build`,
 `run` and `dev` first run `script/assets`, and `build` and `version` both
@@ -1245,6 +1246,8 @@ We provide:
 - `script/assets` — extract Alpine.js from its tarball in `3p/` (see
   [Third-party browser assets](#third-party-browser-assets))
 - `script/test` — run the test suite
+- `script/test-browser` — run the browser test in Docker (see
+  [Third-party browser assets](#third-party-browser-assets))
 - `script/lint` — run golangci-lint in Docker (see Linting below)
 - `script/fmt` — format all code (writes)
 - `script/fmt-check` — check formatting (read-only)
@@ -1265,9 +1268,29 @@ 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 webhook page and the event log
+under the real policy and checks that: both add forms stay hidden until Add is
+clicked; choosing Slack in the add target form leaves the HTTP fields out of
+what it submits; an event expands and collapses, and so do a delivery's
+attempts inside it; and at phone width the menu button opens and closes the
+mobile menu. It also fails if the browser reports a console warning or error,
+an uncaught exception, or anything the policy refused. It is not part of
+`make test`, `make check` or the image build (its file is built only with the
+`browser` build tag). Run it with `make test-browser` after changing
+`templates/` or `static/js/`: that builds `Dockerfile.browser`, which runs the
+test in a digest-pinned headless browser image, so the host needs no browser.
+
+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.
@@ -1280,10 +1303,11 @@ 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
 
@@ -2764,7 +2788,7 @@ 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/
@@ -2857,13 +2881,14 @@ 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
 ├── Dockerfile.lint            # Lint-only image built by script/lint
-├── Makefile                   # 10 of 17 targets shim script/; 7 are inline
+├── Dockerfile.browser         # Browser test image built by script/test-browser
+├── Makefile                   # 11 of 18 targets shim script/; 7 are inline
 ├── go.mod / go.sum
 └── .golangci.yml              # Linter configuration
 ```