Cache radar tile images with a Service Worker

public/sw.js: network-falling-back-to-cache for RainViewer radar tile
requests specifically (nothing else — the app itself is served by the
same local server that's already the offline resource in the van, so
there's no app-shell to precache). Registered from layout.erb, a no-op
where it can't register (insecure context) rather than an error.

Verified: registers and controls the page, tiles get cached on a
normal fetch, cache.match() correctly retrieves them by the same
request. CDP's network-blocking simulation doesn't interact cleanly
with Service Worker fetch interception (a known tooling quirk, not
specific to this), so the actual offline fallback path is the
standard try/catch pattern, verified by code review rather than a
simulated-offline screenshot.

public/sw.js

@@ -0,0 +1,63 @@
+// Caches RainViewer radar tile images so the last-seen frames stay visible
+// once the network is actually gone — nothing else here. No app-shell
+// precaching: the app itself is served by the same local server that's
+// already the offline resource in the van, so there's no "app unreachable
+// but should still work" scenario to solve, unlike the radar tiles (a
+// third-party CDN).
+const CACHE_VERSION = "v1";
+const RADAR_CACHE = `radar-tiles-${CACHE_VERSION}`;
+
+// ponytail: a rough cap, not real LRU/expiry — trims oldest-by-insertion via
+// Cache API iteration order (stable in practice, not spec-guaranteed).
+// Revisit with timestamp-based expiry if storage actually becomes a
+// problem; 400 tiles is at most a few MB either way.
+const MAX_RADAR_TILES = 400;
+
+function isRadarTile(url) {
+  return url.hostname.endsWith("rainviewer.com") && url.pathname.includes("/v2/radar/");
+}
+
+self.addEventListener("install", (event) => {
+  self.skipWaiting();
+});
+
+self.addEventListener("activate", (event) => {
+  event.waitUntil(
+    caches
+      .keys()
+      .then((keys) => Promise.all(keys.filter((key) => key !== RADAR_CACHE).map((key) => caches.delete(key))))
+      .then(() => self.clients.claim()),
+  );
+});
+
+self.addEventListener("fetch", (event) => {
+  const url = new URL(event.request.url);
+  if (!isRadarTile(url)) return;
+
+  event.respondWith(networkFallingBackToCache(event.request));
+});
+
+// Network first (frame ids rotate as new radar sweeps come in, so a cached
+// tile can go stale) — cache is only the answer once the network actually
+// fails, not a shortcut when it's merely slow.
+async function networkFallingBackToCache(request) {
+  const cache = await caches.open(RADAR_CACHE);
+  try {
+    const response = await fetch(request);
+    if (response.ok) {
+      cache.put(request, response.clone());
+      trimRadarCache(cache);
+    }
+    return response;
+  } catch (err) {
+    const cached = await cache.match(request);
+    if (cached) return cached;
+    throw err;
+  }
+}
+
+async function trimRadarCache(cache) {
+  const keys = await cache.keys();
+  if (keys.length <= MAX_RADAR_TILES) return;
+  await Promise.all(keys.slice(0, keys.length - MAX_RADAR_TILES).map((key) => cache.delete(key)));
+}