Installable as an app (PWA): manifests, offline-only service worker, icons

internal/web/assets/offline.html

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+<!doctype html>
+<html lang="en">
+<head>
+  <meta charset="utf-8">
+  <meta name="viewport" content="width=device-width, initial-scale=1">
+  <meta name="theme-color" content="#1b1e25">
+  <!-- Served in place of whatever page failed to load, so a refresh retries that page. -->
+  <meta http-equiv="refresh" content="15">
+  <title>Offline</title>
+  <!-- Self-contained: nothing else is cached, so styles.css and the fonts can't load from here. -->
+  <style>
+    html, body { margin: 0; height: 100%; }
+    body {
+      display: grid;
+      place-items: center;
+      padding: 16px;
+      background: #14161b;
+      color: #e6e8ee;
+      font: 16px/1.5 system-ui, sans-serif;
+      color-scheme: dark;
+    }
+    main {
+      max-width: 360px;
+      padding: 32px 24px;
+      text-align: center;
+      background: #1b1e25;
+      border: 1px solid #2e333f;
+      border-radius: 12px;
+    }
+    svg { width: 48px; height: 48px; }
+    h1 { margin: 12px 0 4px; font-size: 22px; }
+    p { margin: 0 0 20px; color: #8a91a0; }
+    a {
+      display: inline-block;
+      padding: 8px 18px;
+      background: #fe113d;
+      color: #fff;
+      border-radius: 8px;
+      font-weight: 600;
+      text-decoration: none;
+    }
+  </style>
+</head>
+<body>
+  <main>
+    <svg viewBox="0 0 32 32" aria-hidden="true">
+      <path d="M16 2 28 7v9c0 7-5 12-12 14C9 28 4 23 4 16V7z" fill="none" stroke="#fe113d" stroke-width="2.5"/>
+      <path d="M11 16h10M16 11v10" stroke="#e6e8ee" stroke-width="2.5" stroke-linecap="round"/>
+    </svg>
+    <h1>Can't reach the panel</h1>
+    <p>Check your connection. This page tries again by itself every few seconds.</p>
+    <a href="">Try again</a>
+  </main>
+</body>
+</html>