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.