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/badge.js

@@ -0,0 +1,12 @@
+export async function setStreakBadge(streak) {
+  if (!("setAppBadge" in navigator)) return;
+  try {
+    if (streak > 0) await navigator.setAppBadge(streak);
+    else await navigator.clearAppBadge();
+  } catch { /* unsupported */ }
+}
+
+export async function clearBadge() {
+  if (!("clearAppBadge" in navigator)) return;
+  try { await navigator.clearAppBadge(); } catch { /* noop */ }
+}