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.

TODO.adoc

@@ -16,7 +16,7 @@ independence from connectivity.
 * [ ] Import Park4Night / iOverlander / Freecampsites data
 * [ ] Kiwix/ZIM offline Wikipedia linked to POI GPS coordinates
 * [x] Cache weather forecast when temporarily connected — Open-Meteo 3-day forecast in the Radar flyout, last successful fetch cached in localStorage as a fallback when offline
-** [ ] Cache the radar tile *images* too, not just the forecast text — needs a Service Worker (own bigger task: registration, cache strategy, invalidation); app has HTTPS everywhere (reverse proxy, incl. the in-van local server) so it's viable, just not done yet
+** [x] Cache the radar tile *images* too — `public/sw.js`, network-falling-back-to-cache for RainViewer tile requests, registered from layout.erb
 
 == NPS visual style