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/en.i18n.json

@@ -351,6 +351,8 @@
     "wanted": "Wanted",
     "shareMine": "Share my seeds",
     "sharedCount": "Shared {n} seeds nearby",
-    "nothingToShare": "Mark some seeds to give, swap or sell first"
+    "nothingToShare": "Mark some seeds to give, swap or sell first",
+    "useLocation": "Use my approximate location",
+    "locationFailed": "Couldn't get your location"
   }
 }