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/hooks/use-connection-epoch.ts

@@ -0,0 +1,33 @@
+"use client";
+
+import { useEffect, useState } from "react";
+
+/**
+ * Changes after a reason the socket may have died: the browser went back
+ * online, the page was restored, or a hidden tab became visible again.
+ * twui's `useWebSocket` gives up after three attempts, so the socket is
+ * remounted instead of waiting for a reload.
+ */
+export default function useConnectionEpoch() {
+    const [epoch, setEpoch] = useState(0);
+
+    useEffect(() => {
+        const bump = () => setEpoch((value) => value + 1);
+
+        const onVisible = () => {
+            if (document.visibilityState == "visible") bump();
+        };
+
+        window.addEventListener("online", bump);
+        window.addEventListener("pageshow", bump);
+        document.addEventListener("visibilitychange", onVisible);
+
+        return () => {
+            window.removeEventListener("online", bump);
+            window.removeEventListener("pageshow", bump);
+            document.removeEventListener("visibilitychange", onVisible);
+        };
+    }, []);
+
+    return epoch;
+}