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/test/ui/market_screen_test.dart

@@ -6,6 +6,7 @@ import 'package:flutter_bloc/flutter_bloc.dart';
 import 'package:flutter_localizations/flutter_localizations.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:tane/i18n/strings.g.dart';
+import 'package:tane/services/coarse_location.dart';
 import 'package:tane/services/social_service.dart';
 import 'package:tane/services/social_settings.dart';
 import 'package:tane/state/offers_cubit.dart';
@@ -13,6 +14,16 @@ import 'package:tane/ui/market_screen.dart';
 
 import '../support/test_support.dart';
 
+/// A location provider returning a fixed coarse position.
+class FakeLocation implements CoarseLocationProvider {
+  FakeLocation(this.lat, this.lon);
+  final double lat;
+  final double lon;
+  @override
+  Future<({double lat, double lon})?> currentCoarseLatLon() async =>
+      (lat: lat, lon: lon);
+}
+
 /// In-memory offer transport: discover matches by geohash prefix.
 class FakeOfferTransport implements OfferTransport {
   final List<Offer> offers;
@@ -120,4 +131,40 @@ void main() {
     expect(find.text('Seeds near you'), findsOneWidget); // app bar title
     expect(find.text('Set up sharing'), findsOneWidget); // offline prompt
   });
+
+  testWidgets('"use my location" fills the area with a coarse geohash',
+      (tester) async {
+    final social = await SocialService.fromRootSeedHex('00' * 32);
+    final settings = SocialSettings(InMemorySecretStore());
+    LocaleSettings.setLocaleSync(AppLocale.en);
+
+    await tester.pumpWidget(
+      TranslationProvider(
+        child: MaterialApp(
+          locale: AppLocale.en.flutterLocale,
+          supportedLocales: AppLocaleUtils.supportedLocales,
+          localizationsDelegates: const [
+            GlobalMaterialLocalizations.delegate,
+            GlobalWidgetsLocalizations.delegate,
+            GlobalCupertinoLocalizations.delegate,
+          ],
+          home: MarketScreen(
+            social: social,
+            settings: settings,
+            location: FakeLocation(41.39, 2.16), // Barcelona-ish
+          ),
+        ),
+      ),
+    );
+    await tester.pumpAndSettle();
+
+    await tester.tap(find.byKey(const Key('market.config')));
+    await tester.pumpAndSettle();
+    await tester.tap(find.byKey(const Key('market.useLocation')));
+    await tester.pumpAndSettle();
+
+    final area = tester.widget<TextField>(find.byKey(const Key('market.area')));
+    expect(area.controller!.text, hasLength(5));
+    expect(area.controller!.text, matches(RegExp(r'^[0-9a-z]+$')));
+  });
 }