Make the app an installable PWA with phone layout and Web Push

Add a generated manifest, icon set, offline page and service worker that never caches authenticated HTML, plus install and update UI. Turn the rail into a drawer below md with container-query internals, dvh and safe-area handling, and reconnect the socket after visibility/network changes. Add Web Push subscriptions, presence-aware delivery, notification settings and tests.

src/utils/url-base64-to-uint8-array.ts

@@ -0,0 +1,18 @@
+/**
+ * A base64url VAPID public key as the bytes `PushManager.subscribe`
+ * expects.
+ */
+export default function urlBase64ToUint8Array(base64_string: string) {
+    const padding = "=".repeat((4 - (base64_string.length % 4)) % 4);
+    const base64 = (base64_string + padding)
+        .replace(/-/g, "+")
+        .replace(/_/g, "/");
+    const raw = atob(base64);
+    const bytes = new Uint8Array(raw.length);
+
+    for (let index = 0; index < raw.length; index += 1) {
+        bytes[index] = raw.charCodeAt(index);
+    }
+
+    return bytes;
+}