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/ui/market_screen.dart

@@ -4,6 +4,7 @@ import 'package:flutter_bloc/flutter_bloc.dart';
 
 import '../data/variety_repository.dart';
 import '../i18n/strings.g.dart';
+import '../services/coarse_location.dart';
 import '../services/social_service.dart';
 import '../services/social_settings.dart';
 import '../state/offers_cubit.dart';
@@ -13,11 +14,20 @@ import 'theme.dart';
 /// an offer transport lazily; local-first, so it degrades to a "set up sharing"
 /// prompt when no relay/area is configured or the network is unreachable.
 class MarketScreen extends StatefulWidget {
-  const MarketScreen({required this.social, required this.settings, super.key});
+  const MarketScreen({
+    required this.social,
+    required this.settings,
+    this.location,
+    super.key,
+  });
 
   final SocialService social;
   final SocialSettings settings;
 
+  /// Optional device-location source for the "use my location" shortcut; when
+  /// null (tests, or a platform without it) the shortcut is hidden.
+  final CoarseLocationProvider? location;
+
   @override
   State<MarketScreen> createState() => _MarketScreenState();
 }
@@ -53,7 +63,8 @@ class _MarketScreenState extends State<MarketScreen> {
     final changed = await showModalBottomSheet<bool>(
       context: context,
       isScrollControlled: true,
-      builder: (_) => _ConfigSheet(settings: widget.settings),
+      builder: (_) =>
+          _ConfigSheet(settings: widget.settings, location: widget.location),
     );
     if (changed == true) await _init();
   }
@@ -325,9 +336,10 @@ class _EmptyState extends StatelessWidget {
 /// Coarse-area + community-server setup. Kept behind progressive disclosure — a
 /// power-user surface — with human-worded labels.
 class _ConfigSheet extends StatefulWidget {
-  const _ConfigSheet({required this.settings});
+  const _ConfigSheet({required this.settings, this.location});
 
   final SocialSettings settings;
+  final CoarseLocationProvider? location;
 
   @override
   State<_ConfigSheet> createState() => _ConfigSheetState();
@@ -356,6 +368,22 @@ class _ConfigSheetState extends State<_ConfigSheet> {
     if (mounted) Navigator.of(context).pop(true);
   }
 
+  /// Fills the area from the device's approximate location, reduced to a coarse
+  /// geohash so no precise fix is stored.
+  Future<void> _useLocation() async {
+    final provider = widget.location;
+    if (provider == null) return;
+    final messenger = ScaffoldMessenger.of(context);
+    final t = context.t;
+    final loc = await provider.currentCoarseLatLon();
+    if (!mounted) return;
+    if (loc == null) {
+      messenger.showSnackBar(SnackBar(content: Text(t.market.locationFailed)));
+      return;
+    }
+    setState(() => _area.text = Geohash.encode(loc.lat, loc.lon, precision: 5));
+  }
+
   @override
   void dispose() {
     _area.dispose();
@@ -402,6 +430,16 @@ class _ConfigSheetState extends State<_ConfigSheet> {
                     helperMaxLines: 3,
                   ),
                 ),
+                if (widget.location != null)
+                  Align(
+                    alignment: AlignmentDirectional.centerStart,
+                    child: TextButton.icon(
+                      key: const Key('market.useLocation'),
+                      icon: const Icon(Icons.my_location, size: 18),
+                      label: Text(t.market.useLocation),
+                      onPressed: _useLocation,
+                    ),
+                  ),
                 const SizedBox(height: 8),
                 // The technical web address lives under progressive disclosure —
                 // most people won't have one, and jargon shouldn't greet them.