weather: cache forecasts and cut redundant loads

[dharmapoudel]

The weather app refetched open-meteo on every mount, every 15 minutes, and every config write -- flashing "loading weather..." each time and hitting the network even when nothing changed.

What this does

• Caches the forecast in localStorage for 15 minutes (the refresh cadence), keyed by rounded coords + units. Warm starts paint instantly with no network; driving to a new city or switching metric/imperial refetches. Corrupt or unavailable storage degrades to the old behavior.
• Background refreshes keep showing the current data instead of flashing "loading". A failed background refresh keeps the stale data instead of swapping to the error screen; the next 15-minute tick retries. (Deliberate behavior change -- happy to revert to error-on-failure if you prefer.)
• Config-change reloads are filtered to the units/location keys instead of firing on every config write.
• Concurrent loads are deduped: a config change racing the initial load shares one fetch.

Files

• packages/webapps/catalog/weather/src/App.tsx -- readForecastCache/writeForecastCache helpers, cache lookup in the load path, in-flight dedupe, key-filtered onChanged.

Validation

• tsc --noEmit clean, vite build clean.
• Logic tests for the cache helpers: hit, miss on coords/units change, stale-entry boundary, corrupt JSON, coord rounding, storage-quota failure -- 9/9 pass.
• Not tested on physical Car Thing hardware.