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/index.html

@@ -985,6 +985,13 @@
                   <span class="slider"></span>
                 </label>
               </div>
+              <div class="settings-item" title="Show the response viewer (eye) button in header">
+                <span class="settings-item-label">Response Viewer</span>
+                <label class="switch switch-sm">
+                  <input type="checkbox" id="appSettingsShowResponseViewer">
+                  <span class="slider"></span>
+                </label>
+              </div>
               <div class="settings-item" title="Show the multi-monitor button in the header (opens Codeman spanned across all displays)">
                 <span class="settings-item-label">Multi-monitor Button</span>
                 <label class="switch switch-sm">