Stop phones zooming when a field is focused

- Viewport: maximum-scale=1 and user-scalable=no (app-like, no pinch or
  double-tap zoom on phones).
- Mobile-only rule keeps every input/textarea/select at 16px so iOS
  Safari's focus auto-zoom never triggers (it ignores user-scalable=no for
  that). Deliberately unlayered so it beats the per-component Tailwind
  sizes; desktop keeps the designed 12-14px fields.

New zoom suite (6 checks): viewport meta, gate input, settings key input,
notes textarea and every study-feed field at >= 16px on a phone, desktop
sizes unchanged. Install sheet 64/64, pwa 16/16, final-ux 12/12,
security 8/8, contrast 20/20.

app/layout.tsx

@@ -41,7 +41,11 @@ export const metadata: Metadata = {
 export const viewport: Viewport = {
   width: "device-width",
   initialScale: 1,
-  maximumScale: 5,
+  // Phones: no pinch/double-tap zoom (the app-like feel). The input font-size
+  // rule in globals.css is what actually stops iOS Safari's focus auto-zoom,
+  // which ignores user-scalable=no.
+  maximumScale: 1,
+  userScalable: false,
   viewportFit: "cover",
   themeColor: [
     { media: "(prefers-color-scheme: light)", color: "#f8fafc" },