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/i18n/strings_en.g.dart

@@ -1114,6 +1114,12 @@ class Translations$market$en {
 
 	/// en: 'Mark some seeds to give, swap or sell first'
 	String get nothingToShare => 'Mark some seeds to give, swap or sell first';
+
+	/// en: 'Use my approximate location'
+	String get useLocation => 'Use my approximate location';
+
+	/// en: 'Couldn't get your location'
+	String get locationFailed => 'Couldn\'t get your location';
 }
 
 // Path: intro.slides
@@ -1887,6 +1893,8 @@ extension on Translations {
 			'market.shareMine' => 'Share my seeds',
 			'market.sharedCount' => ({required Object n}) => 'Shared ${n} seeds nearby',
 			'market.nothingToShare' => 'Mark some seeds to give, swap or sell first',
+			'market.useLocation' => 'Use my approximate location',
+			'market.locationFailed' => 'Couldn\'t get your location',
 			_ => null,
 		};
 	}