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/auth.js

@@ -0,0 +1,17 @@
+const TOKEN_KEY = "tym_token";
+
+export function getToken() {
+  return localStorage.getItem(TOKEN_KEY);
+}
+
+export function setToken(token) {
+  localStorage.setItem(TOKEN_KEY, token);
+}
+
+export function clearToken() {
+  localStorage.removeItem(TOKEN_KEY);
+}
+
+export function isLoggedIn() {
+  return !!getToken();
+}