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_test.go

@@ -747,10 +747,10 @@ func TestNoCache_SetsHeaders(t *testing.T) {
 
 // --- SecurityHeaders Middleware Tests ---
 
-// hstsHeaders serves one request through SecurityHeaders built over
-// a Config with the given HSTSMaxAge, and returns every
-// Strict-Transport-Security value on the response.
-func hstsHeaders(t *testing.T, maxAge time.Duration) []string {
+// securityHeaders serves one request through SecurityHeaders built
+// over a Config with the given HSTSMaxAge, and returns the response's
+// headers.
+func securityHeaders(t *testing.T, maxAge time.Duration) http.Header {
 	t.Helper()
 
 	cfg := &config.Config{HSTSMaxAge: maxAge}
@@ -772,7 +772,7 @@ func hstsHeaders(t *testing.T, maxAge time.Duration) []string {
 
 	handler.ServeHTTP(w, req)
 
-	return w.Header().Values("Strict-Transport-Security")
+	return w.Header()
 }
 
 func TestSecurityHeaders_HSTS(t *testing.T) {
@@ -804,11 +804,28 @@ func TestSecurityHeaders_HSTS(t *testing.T) {
 		t.Run(tt.name, func(t *testing.T) {
 			t.Parallel()
 
-			assert.Equal(t, tt.expected, hstsHeaders(t, tt.maxAge))
+			assert.Equal(
+				t, tt.expected,
+				securityHeaders(t, tt.maxAge).Values(
+					"Strict-Transport-Security",
+				),
+			)
 		})
 	}
 }
 
+// TestSecurityHeaders_CSP pins the Content-Security-Policy: a page
+// takes its scripts and styles only from files on its own origin,
+// never from an inline script, event handler or style.
+func TestSecurityHeaders_CSP(t *testing.T) {
+	t.Parallel()
+
+	assert.Equal(
+		t, "default-src 'self'; script-src 'self'; style-src 'self'",
+		securityHeaders(t, 0).Get("Content-Security-Policy"),
+	)
+}
+
 // --- MaxBodySize Middleware Tests ---
 
 const testBodyLimit int64 = 64