feat(site-builder): webhook destination controls on the contact form

ContactForm gains four craft props (destinationType, webhookUrl,
webhookSecretId, webhookAuthMode), all present in craft.props defaults so
FormStylePanel's `nodeProps.X !== undefined` gates actually render their
controls. The controls live in FormStylePanel (RightPanel renders only
GuidedStyles, so related.settings would be dead UI).

relayFormWiring widens the marker to optionally carry type/url/secret/
authmode BETWEEN `id` and `recipient`, which is where FormRelayRewrite.php's
parser looks. A marker with no type is byte-identical to what shipped before
-- pinned by a test that diffs an explicit-email form against one with no
destination props at all, since every already-published site depends on that
shape continuing to provision an email endpoint.

Every optional attribute value goes through one escaping site (markerAttr ->
escapeAttr); type and authmode are additionally allowlisted, so a case-drifted
"Bearer" reaches the relay as the exact literal it compares against instead of
being silently downgraded to unsigned.

The raw shared secret is never a prop: it is held in WebhookSecretField's
local state, POSTed to /api/form-webhook-secret.php on blur, and only the
returned opaque id is persisted. The field is write-only (set / replace /
remove, never view) because the endpoint has no read route, and the endpoint's
429 cap message is surfaced verbatim so a customer can act on it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

craft/src/components/forms/ContactForm.toHtml.test.ts

@@ -196,3 +196,83 @@ describe('ContactForm.craft.props includes animation/visibility defaults', () =>
     });
   });
 });
+
+/* ---------- Webhook destination (Task 10) ----------
+   This file builds props inline rather than spreading a shared object, so
+   `defaultProps` is introduced here for the destination cases only; every
+   pre-existing test above is untouched. */
+const defaultProps = { fields: [] as any[], formAction: '#' };
+
+describe('ContactForm.craft.props includes the destination defaults', () => {
+  // The trap this pins: FormStylePanel renders each destination control behind
+  // `nodeProps.X !== undefined`, so a prop omitted from these defaults yields an
+  // invisible control and the whole feature looks like it does nothing.
+  test('destinationType/webhookUrl/webhookSecretId/webhookAuthMode are all present', () => {
+    expect(ContactForm.craft!.props).toMatchObject({
+      destinationType: 'email',
+      webhookUrl: '',
+      webhookSecretId: '',
+      webhookAuthMode: 'signature',
+    });
+  });
+
+  test('no craft prop holds a raw secret -- only an id', () => {
+    const keys = Object.keys(ContactForm.craft!.props as object);
+    expect(keys).toContain('webhookSecretId');
+    expect(keys.filter((k) => /secret/i.test(k))).toEqual(['webhookSecretId']);
+  });
+});
+
+describe('ContactForm.toHtml destination marker', () => {
+  test('email destination emits the legacy marker unchanged', () => {
+    const out = toHtml(
+      { ...defaultProps, destinationType: 'email', recipientEmail: 'a@example.com', thankYouUrl: '' }, '');
+    expect(out.html).toContain('<!--WHP-FORM');
+    expect(out.html).toContain('recipient="a@example.com"');
+    expect(out.html).not.toContain('type="webhook"');
+  });
+
+  test('BYTE-IDENTITY: an email destination emits exactly what a pre-feature form emits', () => {
+    // The guarantee every already-published site depends on: a marker with no
+    // `type` still provisions an email endpoint, so its bytes must not drift by
+    // so much as a space. Compared against a form that has no destination props
+    // at all (a page saved before this feature existed).
+    const legacy = toHtml({ ...defaultProps, recipientEmail: 'a@example.com', thankYouUrl: '/thx' }, '', 'n1');
+    const explicit = toHtml(
+      { ...defaultProps, destinationType: 'email', webhookUrl: '', webhookSecretId: '',
+        webhookAuthMode: 'signature', recipientEmail: 'a@example.com', thankYouUrl: '/thx' }, '', 'n1');
+    expect(explicit.html).toBe(legacy.html);
+    // ...and the marker itself is the exact narrow shape, anchored.
+    expect(legacy.html).toMatch(/^<!--WHP-FORM id="F_[0-9a-z]+" recipient="a@example\.com" thankyou="\/thx"--><form /);
+  });
+
+  test('webhook destination emits type, url, secret id and auth mode', () => {
+    const out = toHtml(
+      { ...defaultProps, destinationType: 'webhook', webhookUrl: 'https://hooks.example.com/x',
+        webhookSecretId: 'sec-1', webhookAuthMode: 'bearer', recipientEmail: 'fb@example.com' }, '');
+    expect(out.html).toContain('type="webhook"');
+    expect(out.html).toContain('url="https://hooks.example.com/x"');
+    expect(out.html).toContain('secret="sec-1"');
+    expect(out.html).toContain('authmode="bearer"');
+    expect(out.html).toContain('recipient="fb@example.com"');
+  });
+
+  test('a raw secret value is never emitted, only its id', () => {
+    const out = toHtml(
+      { ...defaultProps, destinationType: 'webhook', webhookUrl: 'https://hooks.example.com/x',
+        webhookSecretId: 'sec-1', webhookSecret: 'SUPERSECRET' } as any, '');
+    // Non-vacuous: the marker IS emitted (so there is something that could have
+    // carried the secret) and carries the id, but not the value.
+    expect(out.html).toContain('secret="sec-1"');
+    expect(out.html).not.toContain('SUPERSECRET');
+  });
+
+  test('a webhook form with no fallback email still emits a marker (never a bare formAction)', () => {
+    const out = toHtml(
+      { ...defaultProps, destinationType: 'webhook', webhookUrl: 'https://hooks.example.com/x' }, '');
+    expect(out.html).toContain('type="webhook"');
+    expect(out.html).toContain('recipient=""');
+    expect(out.html).toMatch(/action="__WHP_FORM_ACTION__F_[0-9a-z]+__"/);
+    expect(out.html).toContain('name="_gotcha"');
+  });
+});