fix(site-builder): review findings on the webhook destination controls

I-1: WebhookSecretField now mounts with key={selectedId}. GuidedStyles renders
FormStylePanel with no key, so a selection change re-rendered rather than
remounted it and React kept `draft`/`status`. Since a failed store deliberately
retains the draft, clicking a second contact form showed node A's raw secret in
node B's field, and the next blur assigned the returned secret_id to the wrong
form (burning a slot against the per-site cap). The status banner leaked the
same way. Both pinned by the reviewer's repro sequence.

I-2: the button is "Clear", not "Remove", and says so -- nothing deletes a
stored key file, so the 50-per-site cap counts stores-ever. Labelling it Remove
told the customer they had reclaimed a slot right up until the 429 that said
otherwise. Actually deleting the file is Task 8's territory.

M-1: inline warning when the webhook URL is blank or not absolute https. The
publish step does refuse these, but into an error_log the customer never reads.
Warning only -- isHttpsWebhookUrl never edits the value or blocks the publish,
since either would trade a loud server-side refusal for a silently inert form.

M-2: the BYTE-IDENTITY test now compares against a literal marker captured by
executing the emitter at 071f3447, not against another head-revision output. The
self-comparison could only catch a drift affecting one side; a uniform one
passed it. Verified: a uniform `<!-- WHP-FORM` drift now fails this assertion.

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

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

@@ -235,15 +235,25 @@ describe('ContactForm.toHtml destination marker', () => {
   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).
+    // so much as a space.
+    //
+    // FROZEN LITERAL, not a self-comparison. Comparing two head-revision outputs
+    // to each other only catches a drift that affects ONE of them -- a uniform
+    // change passes it. This string was captured from `071f3447` (the revision
+    // deployed to production before this feature) and is the actual reference:
+    // if it has to be edited, every already-published site's forms have changed
+    // shape and that is the thing to stop, not the test.
+    const FROZEN_LEGACY_MARKER =
+      '<!--WHP-FORM id="F_3hodg" recipient="a@example.com" thankyou="/thx"-->';
+
     const legacy = toHtml({ ...defaultProps, recipientEmail: 'a@example.com', thankYouUrl: '/thx' }, '', 'n1');
+    expect(legacy.html.startsWith(`${FROZEN_LEGACY_MARKER}<form `)).toBe(true);
+
+    // ...and the new props, set to their defaults, change nothing about it.
     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', () => {