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.
