fix(block2): sharing sheet overflow + location button feedback/robustness

Reported: sheet overflowed 7.7px with the keyboard up, and 'use my location'
seemed to do nothing.
- Config sheet is now scrollable (SingleChildScrollView) — no keyboard overflow.
- 'Use my location' shows an inline spinner + inline error (the old snackbar was
  hidden BEHIND the bottom sheet, so failures were invisible). Message is now
  actionable ('check location is on and the permission is granted').
- Provider hardening: request permission first; on permanent denial open app
  settings; if location services are off open location settings; time-limit the
  fix and fall back to last-known position so it doesn't hang indoors.

Analyzer clean; market tests green.

apps/app_seeds/lib/services/coarse_location.dart

@@ -16,23 +16,38 @@ class GeolocatorCoarseLocation implements CoarseLocationProvider {
   @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) {
+      // Give the user a way out of a permanent denial.
+      if (permission == LocationPermission.deniedForever) {
+        await Geolocator.openAppSettings();
         return null;
       }
-      final position = await Geolocator.getCurrentPosition(
-        locationSettings: const LocationSettings(
-          accuracy: LocationAccuracy.low,
-        ),
-      );
+      if (permission == LocationPermission.denied) return null;
+      if (!await Geolocator.isLocationServiceEnabled()) {
+        await Geolocator.openLocationSettings();
+        return null;
+      }
+
+      // A fresh coarse fix can take a while (or never come indoors); fall back
+      // to the last known position so the button stays responsive.
+      Position? position;
+      try {
+        position = await Geolocator.getCurrentPosition(
+          locationSettings: const LocationSettings(
+            accuracy: LocationAccuracy.low,
+            timeLimit: Duration(seconds: 12),
+          ),
+        );
+      } catch (_) {
+        position = await Geolocator.getLastKnownPosition();
+      }
+      if (position == null) return null;
       return (lat: position.latitude, lon: position.longitude);
     } catch (_) {
-      return null; // service off, plugin unsupported (desktop), timeout, …
+      return null; // plugin unsupported (desktop), etc.
     }
   }
 }