feat: add mobile detection with friendly unsupported message

Detect mobile viewport (innerWidth < 768) at init and show a centered
'Not yet available on mobile.' box instead of starting the full monitor.
No polling, no host rows, no sparklines on mobile.

Desktop behavior is completely unchanged.

Ref #2, ref #4

src/main.js

@@ -1131,6 +1131,25 @@ function handleResize(state) {
 async function init() {
     log.info("NetWatch starting");
 
+    // Mobile detection — show a friendly message and bail out early
+    if (window.innerWidth < 768) {
+        const app = document.getElementById("app");
+        app.innerHTML = `
+        <div class="mx-auto px-[5%] py-8">
+          <header class="mb-8">
+            <h1 class="text-3xl font-bold text-white"><a href="https://git.eeqj.de/sneak/netwatch" target="_blank" rel="noopener" class="underline decoration-dashed decoration-gray-500 underline-offset-4">NetWatch</a> by <a href="https://sneak.berlin" target="_blank" rel="noopener" class="text-blue-400 underline hover:text-blue-300">@sneak</a></h1>
+            <p class="text-gray-400 mt-2">Real-time network connectivity monitor</p>
+          </header>
+          <div style="display:flex;align-items:center;justify-content:center;min-height:50vh;">
+            <div style="background:#1f2937;border:1px solid #374151;border-radius:12px;padding:2rem 1.5rem;text-align:center;max-width:90vw;">
+              <p style="font-size:1.25rem;color:#e5e7eb;margin:0;">Not yet available on mobile.</p>
+            </div>
+          </div>
+        </div>`;
+        log.info("Mobile viewport detected — skipping monitor startup");
+        return;
+    }
+
     // Probe common gateway IPs to find the local router
     const gateway = await detectGateway();
     const localHosts = [LOCAL_CPE];