diff --git a/public/index.html b/public/index.html
index d104441..7a44cbb 100644
--- a/public/index.html
+++ b/public/index.html
@@ -69,6 +69,7 @@
       linear-gradient(rgba(11,20,26,.95), rgba(11,20,26,.95)); }
     .empty { margin: auto; color: var(--muted); }
     .chat-head { display: flex; gap: 12px; align-items: center; padding: 12px 16px; background: var(--panel); border-bottom: 1px solid #0a0f13; }
+    .back-btn { display: none; background: transparent; color: var(--text); font-size: 26px; line-height: 1; padding: 0 6px; }
     .thread { flex: 1; overflow-y: auto; padding: 20px; display: flex; flex-direction: column; gap: 6px; }
     .bubble { max-width: 65%; padding: 8px 10px 6px; border-radius: 10px; font-size: 14.5px; line-height: 1.35; position: relative; word-wrap: break-word; }
     .bubble.in { background: var(--bubble-in); align-self: flex-start; border-top-left-radius: 2px; }
@@ -84,10 +85,32 @@
     .attach { cursor: pointer; font-size: 20px; user-select: none; }
     .send { background: var(--accent); color: #04140f; font-weight: 600; border-radius: 20px; }
 
+    /* ---- modal ---- */
+    .modal-overlay {
+      position: fixed; inset: 0; background: rgba(0,0,0,.6);
+      display: grid; place-items: center; z-index: 100; padding: 20px;
+    }
+    .modal-card {
+      background: var(--panel); border-radius: 14px; padding: 24px;
+      width: 100%; max-width: 360px; display: flex; flex-direction: column; gap: 14px;
+      box-shadow: 0 20px 60px rgba(0,0,0,.5);
+    }
+    .modal-title { margin: 0; font-size: 18px; }
+    .modal-input {
+      padding: 12px 14px; border-radius: 10px; border: 1px solid var(--panel-2);
+      background: var(--panel-2); color: var(--text); font-size: 15px;
+    }
+    .modal-input:focus { outline: 2px solid var(--accent); }
+    .modal-err { color: #f15c6d; font-size: 13px; min-height: 16px; }
+    .modal-actions { display: flex; gap: 10px; justify-content: flex-end; }
+    .modal-actions button { padding: 10px 16px; }
+
     @media (max-width: 720px) {
       .shell { grid-template-columns: 1fr; }
-      .sidebar { display: none; }
+      /* Sidebar shows by default; hide it once a chat is open so the thread is full-width. */
       .shell.chat-open .sidebar { display: none; }
+      .shell:not(.chat-open) .main { display: none; }
+      .back-btn { display: block; }
     }
   </style>
 </head>
diff --git a/public/js/app.js b/public/js/app.js
index f74a8c2..9a58fca 100644
--- a/public/js/app.js
+++ b/public/js/app.js
@@ -149,33 +149,89 @@ async function refreshContacts() {
   }
 }
 
-async function startNewChat() {
-  const username = prompt("Username to message:");
-  if (!username) return;
-  try {
-    const user = await api.lookup(username.trim().toLowerCase());
-    const contact = {
-      userId: user.userId,
-      username: user.username,
-      displayName: user.displayName,
-      deviceIds: [],
-    };
-    await contacts.put(contact);
-    await refreshContacts();
-    await openChat(contact);
-  } catch (ex) {
-    alert("Could not find user: " + ex.message);
-  }
+function startNewChat() {
+  openModal({
+    title: "New chat",
+    placeholder: "Enter a username",
+    action: "Start chat",
+    onSubmit: async (value, setError, close) => {
+      const uname = value.trim().toLowerCase();
+      if (!uname) return setError("Enter a username");
+      if (uname === state.me.username) return setError("That's you 🙂");
+      try {
+        const user = await api.lookup(uname);
+        const contact = {
+          userId: user.userId,
+          username: user.username,
+          displayName: user.displayName,
+          deviceIds: [],
+        };
+        await contacts.put(contact);
+        await refreshContacts();
+        close();
+        await openChat(contact);
+      } catch (ex) {
+        setError(ex.message === "not found" ? "No user found: @" + uname : ex.message);
+      }
+    },
+  });
+}
+
+// Lightweight in-app modal (replaces window.prompt, which Brave/PWAs suppress).
+function openModal({ title, placeholder, action, onSubmit }) {
+  const input = el("input", { className: "modal-input", placeholder, autocomplete: "off" });
+  const err = el("div", { className: "modal-err" });
+  const cancel = el("button", { type: "button", className: "ghost", textContent: "Cancel" });
+  const submit = el("button", { type: "button", className: "primary", textContent: action });
+  const card = el("div", { className: "modal-card" },
+    el("h2", { className: "modal-title", textContent: title }),
+    input,
+    err,
+    el("div", { className: "modal-actions" }, cancel, submit)
+  );
+  const overlay = el("div", { className: "modal-overlay" }, card);
+  document.body.append(overlay);
+  setTimeout(() => input.focus(), 0);
+
+  const close = () => overlay.remove();
+  const setError = (m) => { err.textContent = m; };
+  const go = async () => {
+    submit.disabled = true;
+    setError("");
+    try {
+      await onSubmit(input.value, setError, close);
+    } finally {
+      submit.disabled = false;
+    }
+  };
+  cancel.onclick = close;
+  submit.onclick = go;
+  overlay.onclick = (e) => { if (e.target === overlay) close(); };
+  input.onkeydown = (e) => {
+    if (e.key === "Enter") { e.preventDefault(); go(); }
+    if (e.key === "Escape") close();
+  };
 }
 
 // ---------- chat view ----------
 async function openChat(contact) {
   state.activeContact = contact;
+  $(".shell")?.classList.add("chat-open");
   const main = $("#main");
   main.innerHTML = "";
 
+  const back = el("button", {
+    className: "back-btn",
+    textContent: "‹",
+    title: "Back",
+    onclick: () => {
+      state.activeContact = null;
+      $(".shell")?.classList.remove("chat-open");
+    },
+  });
   const head = el("div", { className: "chat-head" });
   head.append(
+    back,
     el("div", { className: "avatar", textContent: (contact.displayName || contact.username)[0].toUpperCase() }),
     el("div", {},
       el("div", { className: "chat-name", textContent: contact.displayName || contact.username }),
diff --git a/public/sw.js b/public/sw.js
index 99c7109..84ae8b9 100644
--- a/public/sw.js
+++ b/public/sw.js
@@ -1,7 +1,7 @@
 // Service worker: offline app shell + Web Push wake-up.
 // It never touches message plaintext — decryption happens only in the page.
 
-const CACHE = "cipher-v1";
+const CACHE = "cipher-v2";
 const SHELL = [
   "/",
   "/index.html",