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/schemas.ts

@@ -290,6 +290,7 @@ export const SettingsUpdateSchema = z
     showTokenCount: z.boolean().optional(),
     showCost: z.boolean().optional(),
     showLifecycleLog: z.boolean().optional(),
+    showResponseViewer: z.boolean().optional(),
     showMonitor: z.boolean().optional(),
     showProjectInsights: z.boolean().optional(),
     showFileBrowser: z.boolean().optional(),