feat: add mobile viewport detection with friendly unavailable message

Detect mobile viewport (window.innerWidth < 768) at startup and show a
centered 'Not yet available on mobile' message instead of the full
monitoring UI. All polling, gateway detection, and network requests are
skipped entirely on mobile viewports.

Desktop behavior is completely unchanged — the mobile check is the very
first thing in init() and returns early before any other setup runs.

README.md

@@ -102,6 +102,9 @@ dist/
   false outage)
 - Clickable service URLs
 - Canvas-based sparkline rendering with devicePixelRatio scaling
+- Mobile detection: viewports narrower than 768px show a friendly "not yet
+  available on mobile" message instead of the monitoring UI (no polling or
+  network requests on mobile)
 - Zero runtime dependencies: all resources bundled into build artifacts
 
 ## Deployment
@@ -125,6 +128,8 @@ properties.
 
 ## Limitations
 
+- **Mobile**: Viewports below 768px wide show a static "not yet available"
+  message. The full monitoring UI requires a desktop-width browser.
 - **CORS**: Some hosts may block cross-origin HEAD requests. The app uses
   `no-cors` mode which allows the request but provides opaque responses. Latency
   is still measurable based on request timing.