feat(ui): toggle to hide the response-viewer (eye) header button

The eye button (View last response) was the only header control with
no visibility setting. Add App Settings -> Display -> "Response
Viewer" (showResponseViewer, default on, per-device like the other
header toggles; added to the displayKeys no-cross-device-sync set
along with showLifecycleLog, which was missing from it).

Hiding uses a marker class with higher specificity — the base rule is
display:inline-flex !important, so an inline style cannot override it.

Verified via Playwright on a fresh instance: 8/8 — default visible,
hides live on save, persists across reload, server schema accepts the
key, re-enable restores, mobile storage isolated.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/web/public/styles.css

@@ -8115,6 +8115,12 @@ kbd {
   display: inline-flex !important;
 }
 
+/* "Response Viewer" header toggle (App Settings → Display) — must out-specify
+   the inline-flex !important above */
+.btn-response-viewer-header.btn-response-viewer-header--hidden {
+  display: none !important;
+}
+
 .response-viewer {
   display: none;
   position: fixed;