feat(student): auto-reconnect with backoff + WS-open retry

Replace the manual "Disconnected → Reconnect button" screen with a small
top-banner that retries the WS up to 8 times (500ms → 5s, ~27s budget).
On open, snapshot replay restores the question card and countdown so a
brief network blip is invisible to the student. After the retry budget
exhausts, fall back to a manual "Reload" card.

Same path covers initial WS-open failures (transient TLS hiccups on the
Aliyun edge), since the first connect() and subsequent reconnects share
the schedule. Auth-related closes (1008) still hard-reload immediately
so an invalidated cookie lands on the join form, not in a retry loop.

Submits are now also gated on ws.readyState === OPEN; clicks during a
reconnect are silent no-ops, and the question re-renders fresh once the
server replays state.

static/quiz.js

@@ -21,6 +21,17 @@ const store = {
   deadlineMs: null,
 };
 
+// WS reconnect with exponential backoff. Total budget is ~27s across 8
+// attempts (500ms, 1s, 2s, 4s, then 5s × 4), which covers typical mobile
+// hand-off and Aliyun-edge TLS hiccups without giving up too quickly.
+const RECONNECT = {
+  attempt: 0,
+  maxAttempts: 8,
+  baseDelayMs: 500,
+  maxDelayMs: 5000,
+  timer: null,
+};
+
 let countdownTimer = null;
 
 function fmtScore(value) {
@@ -134,23 +145,83 @@ function connect() {
   const protocol = window.location.protocol === "https:" ? "wss" : "ws";
   const ws = new WebSocket(`${protocol}://${window.location.host}/ws/student/${sid}`);
   store.ws = ws;
+  ws.addEventListener("open", () => {
+    clearReconnectState();
+  });
   ws.addEventListener("message", (event) => {
     try { handleMessage(JSON.parse(event.data)); } catch (e) { console.warn("bad ws msg", e); }
   });
-  ws.addEventListener("close", () => {
-    // session_reset already drove a re-boot; suppress the generic
-    // "disconnected" screen so it doesn't briefly flash on top of the
-    // "Re-joining…" interstitial.
+  ws.addEventListener("close", (event) => {
+    // session_reset already drove a re-boot; suppress the reconnect path
+    // so it doesn't fight with the "Re-joining…" interstitial.
     if (store.resetting) return;
     stopCountdown();
-    setView(`
-      <div class="card narrow center">
-        <h1>Disconnected</h1>
-        <p class="muted">Your connection dropped.</p>
-        <button class="btn primary block" onclick="window.location.reload()">Reconnect</button>
-      </div>
-    `);
+    // 1008 = policy violation (server rejected the cookie / session).
+    // Retrying won't help; reload so /me re-checks auth and we land on
+    // the join form (or "ask your instructor") cleanly.
+    if (event.code === 1008) {
+      window.location.reload();
+      return;
+    }
+    scheduleReconnect();
   });
+  ws.addEventListener("error", () => {
+    // The "close" event will fire next; reconnect handling lives there.
+  });
+}
+
+function scheduleReconnect() {
+  if (store.resetting) return;
+  if (RECONNECT.attempt >= RECONNECT.maxAttempts) {
+    showReconnectFailed();
+    return;
+  }
+  RECONNECT.attempt += 1;
+  const delay = Math.min(
+    RECONNECT.baseDelayMs * Math.pow(2, RECONNECT.attempt - 1),
+    RECONNECT.maxDelayMs
+  );
+  showReconnectingBanner(`Reconnecting… (${RECONNECT.attempt}/${RECONNECT.maxAttempts})`);
+  RECONNECT.timer = setTimeout(connect, delay);
+}
+
+function clearReconnectState() {
+  if (RECONNECT.timer) {
+    clearTimeout(RECONNECT.timer);
+    RECONNECT.timer = null;
+  }
+  RECONNECT.attempt = 0;
+  hideReconnectingBanner();
+}
+
+function showReconnectingBanner(text) {
+  let el = document.querySelector("#reconnect-banner");
+  if (!el) {
+    el = document.createElement("div");
+    el.id = "reconnect-banner";
+    el.className = "reconnect-banner";
+    el.setAttribute("role", "status");
+    el.setAttribute("aria-live", "polite");
+    document.body.appendChild(el);
+  }
+  el.textContent = text;
+  el.hidden = false;
+}
+
+function hideReconnectingBanner() {
+  const el = document.querySelector("#reconnect-banner");
+  if (el) el.hidden = true;
+}
+
+function showReconnectFailed() {
+  hideReconnectingBanner();
+  setView(`
+    <div class="card narrow center">
+      <h1>Disconnected</h1>
+      <p class="muted">We couldn't reconnect after several tries. Reload to try again.</p>
+      <button class="btn primary block" onclick="window.location.reload()">Reload</button>
+    </div>
+  `);
 }
 
 function handleMessage(message) {
@@ -172,6 +243,7 @@ function handleSessionReset() {
   // server) and we'll land cleanly on the join form.
   store.resetting = true;
   stopCountdown();
+  clearReconnectState();
   store.me = null;
   store.currentQuestion = null;
   store.submitted = null;
@@ -243,7 +315,12 @@ function renderQuestion(message) {
 }
 
 function submitAnswer(answer) {
-  if (!store.ws || !store.currentQuestion || store.submitted || store.pickedAnswer) return;
+  if (!store.currentQuestion || store.submitted || store.pickedAnswer) return;
+  // Drop the click silently if the WS isn't open right now (mid-reconnect
+  // or already torn down). On reconnect the server replays question_open
+  // for the same qidx, which re-renders the card with buttons re-enabled,
+  // so the student just clicks again.
+  if (!store.ws || store.ws.readyState !== WebSocket.OPEN) return;
   store.pickedAnswer = answer;
   document.querySelectorAll("[data-answer]").forEach((btn) => {
     btn.disabled = true;

static/style.css

@@ -332,6 +332,26 @@ input:focus, textarea:focus, select:focus {
 .alert.error { margin: 0; }
 .alert.info { border-left-color: var(--primary); color: var(--primary); background: color-mix(in srgb, var(--primary) 5%, transparent); }
 
+/* ---------- Reconnect banner ---------- */
+
+.reconnect-banner {
+  position: fixed;
+  top: 12px;
+  left: 50%;
+  transform: translateX(-50%);
+  z-index: 50;
+  padding: 8px 16px;
+  border-radius: 999px;
+  font-size: 0.85rem;
+  font-weight: 500;
+  color: var(--warn-text);
+  background: var(--warn);
+  box-shadow: var(--shadow);
+  letter-spacing: 0.01em;
+  pointer-events: none;
+}
+.reconnect-banner[hidden] { display: none; }
+
 /* ---------- Admin topbar ---------- */
 
 .admin-body { padding-bottom: 32px; }