feat: PWA update strategy with hashed assets and cache control

Move frontend sources to frontend/, build content-hashed assets at deploy,
network-first SW for shell/JS, immutable cache for /assets/, version.json
update banner, and Cache-Control middleware. Docs in docs/PWA-STRATEGY.md.

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/lib/offlineQueue.js

@@ -0,0 +1,44 @@
+const DB_NAME = "tym-offline";
+const STORE = "queue";
+
+function openDb() {
+  return new Promise((resolve, reject) => {
+    const req = indexedDB.open(DB_NAME, 1);
+    req.onupgradeneeded = () => req.result.createObjectStore(STORE, { keyPath: "id", autoIncrement: true });
+    req.onsuccess = () => resolve(req.result);
+    req.onerror = () => reject(req.error);
+  });
+}
+
+export async function enqueue(entry) {
+  const db = await openDb();
+  return new Promise((resolve, reject) => {
+    const tx = db.transaction(STORE, "readwrite");
+    tx.objectStore(STORE).add({ ...entry, queued_at: Date.now() });
+    tx.oncomplete = () => { db.close(); resolve(); };
+    tx.onerror = () => reject(tx.error);
+  });
+}
+
+export async function flushQueue(apiLog) {
+  const db = await openDb();
+  const entries = await new Promise((resolve, reject) => {
+    const tx = db.transaction(STORE, "readonly");
+    const req = tx.objectStore(STORE).getAll();
+    req.onsuccess = () => resolve(req.result || []);
+    req.onerror = () => reject(req.error);
+  });
+
+  for (const entry of entries) {
+    try {
+      await apiLog({ slot_id: entry.slot_id, status: entry.status, source: "offline" });
+      await new Promise((resolve, reject) => {
+        const tx = db.transaction(STORE, "readwrite");
+        tx.objectStore(STORE).delete(entry.id);
+        tx.oncomplete = resolve;
+        tx.onerror = () => reject(tx.error);
+      });
+    } catch { break; }
+  }
+  db.close();
+}