feat: responsive mobile layout for host rows (closes #2)

Redesign host rows for portrait/mobile viewports (<=768px):
- Host info panel stacks on top, full width
- Sparkline renders full width below
- Each host row becomes taller to accommodate vertical layout
- Summary line wraps gracefully
- Header controls stack below title

Desktop layout is unchanged — all changes are inside a media query.

src/main.js

@@ -1128,42 +1128,9 @@ function handleResize(state) {
 
 // --- Bootstrap ---------------------------------------------------------------
 
-// --- Mobile Detection --------------------------------------------------------
-
-function isMobile() {
-    // Check both user agent and viewport width for robust detection
-    const uaMatch =
-        /Android|webOS|iPhone|iPad|iPod|BlackBerry|IEMobile|Opera Mini/i.test(
-            navigator.userAgent,
-        );
-    const narrowViewport = window.innerWidth <= 768;
-    return uaMatch || narrowViewport;
-}
-
-function buildMobileUI() {
-    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 text-sm mt-2">Real-time network latency monitor</p>
-      </header>
-      <div style="margin: 2rem auto; max-width: 500px; padding: 3rem 2rem; background: rgba(31, 41, 55, 0.7); border: 1px solid rgba(75, 85, 99, 0.5); border-radius: 1rem; text-align: center;">
-        <p style="font-size: 1.5rem; font-weight: 600; color: #d1d5db;">Not yet available on mobile.</p>
-        <p style="font-size: 0.875rem; color: #6b7280; margin-top: 1rem;">Please visit on a desktop browser for the full experience.</p>
-      </div>
-    </div>`;
-}
-
 async function init() {
     log.info("NetWatch starting");
 
-    if (isMobile()) {
-        log.info("Mobile device detected — showing placeholder");
-        buildMobileUI();
-        return;
-    }
-
     // Probe common gateway IPs to find the local router
     const gateway = await detectGateway();
     const localHosts = [LOCAL_CPE];