feat(block2): 'use my location' — set the sharing area without typing a code

Makes the coarse area human, per feedback that a typed code is jargon.
- CoarseLocationProvider interface + geolocator-backed impl (BSD-3): low
  accuracy only, returns null on any denial/error (degrades quietly). Behind an
  interface so it's fakeable and the plugin stays at the composition root.
- Config sheet: a 'Use my approximate location' button (shown only when a
  provider is wired) fills the area from the device position, reduced to a
  5-char geohash via commons_core's Geohash — a precise fix never leaves the
  device. 'Couldn't get your location' on failure.
- Threaded via TaneApp(location); main wires GeolocatorCoarseLocation.
- Platform: ACCESS_COARSE_LOCATION (Android) + NSLocationWhenInUseUsageDescription
  (iOS), both scoped to the coarse-area use.
- i18n en/es/pt. Widget test fills the area from a fake location. 8 tests green.

apps/app_seeds/lib/main.dart

@@ -5,6 +5,7 @@ import 'data/species_repository.dart';
 import 'data/variety_repository.dart';
 import 'di/injector.dart';
 import 'i18n/strings.g.dart';
+import 'services/coarse_location.dart';
 import 'services/onboarding_store.dart';
 import 'services/social_service.dart';
 import 'services/social_settings.dart';
@@ -22,6 +23,7 @@ Future<void> main() async {
         onboarding: onboarding,
         social: getIt<SocialService>(),
         socialSettings: getIt<SocialSettings>(),
+        location: const GeolocatorCoarseLocation(),
         showIntro: !await onboarding.introSeen(),
       ),
     ),