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/app.dart

@@ -7,6 +7,7 @@ import 'package:go_router/go_router.dart';
 import 'data/species_repository.dart';
 import 'data/variety_repository.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';
@@ -31,6 +32,7 @@ class TaneApp extends StatelessWidget {
     required this.onboarding,
     this.social,
     this.socialSettings,
+    this.location,
     this.showIntro = false,
     super.key,
   }) : _router = _buildRouter(
@@ -39,6 +41,7 @@ class TaneApp extends StatelessWidget {
           showIntro,
           social,
           socialSettings,
+          location,
         );
 
   final VarietyRepository repository;
@@ -49,6 +52,9 @@ class TaneApp extends StatelessWidget {
   /// null the market stays a "coming soon" card and `/market` is not routed.
   final SocialService? social;
   final SocialSettings? socialSettings;
+
+  /// Optional device-location source for the market's "use my location".
+  final CoarseLocationProvider? location;
   final bool showIntro;
   final GoRouter _router;
 
@@ -58,6 +64,7 @@ class TaneApp extends StatelessWidget {
     bool showIntro,
     SocialService? social,
     SocialSettings? socialSettings,
+    CoarseLocationProvider? location,
   ) {
     return GoRouter(
       initialLocation: showIntro ? '/intro' : '/',
@@ -70,8 +77,11 @@ class TaneApp extends StatelessWidget {
         if (social != null && socialSettings != null)
           GoRoute(
             path: '/market',
-            builder: (context, state) =>
-                MarketScreen(social: social, settings: socialSettings),
+            builder: (context, state) => MarketScreen(
+              social: social,
+              settings: socialSettings,
+              location: location,
+            ),
           ),
         GoRoute(
           path: '/intro',