Fall back to OpenStreetMap when the build has no Google Maps key

[alex-so-3]

A build without MAPS_API_KEY crashes when the map screen opens (a fork, or a local build without secrets.properties). CONTRIBUTING says the app builds without a key but the map will not load; in practice the app does not start.

When the build has no Maps key and Google Maps is the chosen provider, MapProviderFactory.effectiveProvider falls back to OpenStreetMap, which needs no key. The map screen and the history screen both use it. Builds with a key behave exactly as before, and AMap is untouched.

Verified: on a phone, a debug build without a key: the app starts and shows OSM on the map and in history. ./gradlew testAll and add_strings.py --check pass.

[parawanderer]

Good change. Never had it outright crash myself for whatever reason from CI builds, likely because I just supplied an invalid key for the debug app instead of no key. Either way, the map would render as empty in that case which is annoying. Thanks for testing the no key path. Defaulting to OpenStreetMap is better in both cases (invalid key/no key).

I'll have a model pass to add tests and also make the CI default to OpenStreetMap instead of a Google map that doesn't render.

[parawanderer]

Pushed 8a49cad on top:

• maps_key_default_value, which build-debug.yml writes when the repository secret is missing, now also falls back to OpenStreetMap. CI debug builds from forks drew an empty Google map with it.
• The key decision is a pure providerFor(providerType, key) with a JVM test (MapProviderFactoryTest). The Context overload only reads the key.
• An unset or unknown provider counts as Google, since that is what create() turns it into.
• Removed MapsActivity.checkApiKey(), which repeated the same check, and used imports in place of fully qualified names.

Drafted by Claude Code, reviewed by @parawanderer