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

internal/middleware/middleware.go

@@ -572,7 +572,9 @@ func (s *Middleware) MetricsAuth() func(http.Handler) http.Handler {
 // SecurityHeaders returns middleware that sets production security
 // headers on every response: HSTS (unless HSTS_MAX_AGE is zero),
 // X-Content-Type-Options, X-Frame-Options, CSP, Referrer-Policy, and
-// Permissions-Policy.
+// Permissions-Policy. The CSP takes scripts and styles only from
+// files on this origin, so a template may hold no inline script,
+// event handler or style attribute.
 func (s *Middleware) SecurityHeaders() func(http.Handler) http.Handler {
 	maxAge := s.params.Config.HSTSMaxAge
 	hsts := "max-age=" + strconv.FormatInt(int64(maxAge/time.Second), 10) +
@@ -594,8 +596,8 @@ func (s *Middleware) SecurityHeaders() func(http.Handler) http.Handler {
 			w.Header().Set(
 				"Content-Security-Policy",
 				"default-src 'self'; "+
-					"script-src 'self' 'unsafe-inline'; "+
-					"style-src 'self' 'unsafe-inline'",
+					"script-src 'self'; "+
+					"style-src 'self'",
 			)
 			w.Header().Set(
 				"Referrer-Policy",