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/services/coarse_location.dart

@@ -0,0 +1,38 @@
+import 'package:geolocator/geolocator.dart';
+
+/// Supplies the device's *approximate* location (or null when unavailable or
+/// denied). Behind an interface so the UI is fakeable in tests and the concrete
+/// platform plugin stays at the composition root. The caller immediately reduces
+/// the result to a low-precision geohash — a precise fix never leaves the device.
+abstract interface class CoarseLocationProvider {
+  Future<({double lat, double lon})?> currentCoarseLatLon();
+}
+
+/// [geolocator]-backed implementation. Requests low accuracy only, and returns
+/// null on any denial/error rather than throwing, so the UI can degrade quietly.
+class GeolocatorCoarseLocation implements CoarseLocationProvider {
+  const GeolocatorCoarseLocation();
+
+  @override
+  Future<({double lat, double lon})?> currentCoarseLatLon() async {
+    try {
+      if (!await Geolocator.isLocationServiceEnabled()) return null;
+      var permission = await Geolocator.checkPermission();
+      if (permission == LocationPermission.denied) {
+        permission = await Geolocator.requestPermission();
+      }
+      if (permission == LocationPermission.denied ||
+          permission == LocationPermission.deniedForever) {
+        return null;
+      }
+      final position = await Geolocator.getCurrentPosition(
+        locationSettings: const LocationSettings(
+          accuracy: LocationAccuracy.low,
+        ),
+      );
+      return (lat: position.latitude, lon: position.longitude);
+    } catch (_) {
+      return null; // service off, plugin unsupported (desktop), timeout, …
+    }
+  }
+}