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/ui/market_screen.dart

@@ -410,6 +410,8 @@ class _ConfigSheetState extends State<_ConfigSheet> {
   final _area = TextEditingController();
   final _servers = TextEditingController();
   bool _loading = true;
+  bool _locationBusy = false;
+  String? _locationError;
 
   @override
   void initState() {
@@ -433,16 +435,21 @@ class _ConfigSheetState extends State<_ConfigSheet> {
   /// 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;
+    if (provider == null || _locationBusy) return;
+    setState(() {
+      _locationBusy = true;
+      _locationError = null;
+    });
     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));
+    setState(() {
+      _locationBusy = false;
+      if (loc == null) {
+        _locationError = context.t.market.locationFailed;
+      } else {
+        _area.text = Geohash.encode(loc.lat, loc.lon, precision: 5);
+      }
+    });
   }
 
   @override
@@ -464,7 +471,8 @@ class _ConfigSheetState extends State<_ConfigSheet> {
       ),
       child: _loading
           ? const SizedBox(height: 120, child: Center(child: CircularProgressIndicator()))
-          : Column(
+          : SingleChildScrollView(
+              child: Column(
               mainAxisSize: MainAxisSize.min,
               crossAxisAlignment: CrossAxisAlignment.stretch,
               children: [
@@ -496,9 +504,23 @@ class _ConfigSheetState extends State<_ConfigSheet> {
                     alignment: AlignmentDirectional.centerStart,
                     child: TextButton.icon(
                       key: const Key('market.useLocation'),
-                      icon: const Icon(Icons.my_location, size: 18),
+                      icon: _locationBusy
+                          ? const SizedBox(
+                              width: 16,
+                              height: 16,
+                              child: CircularProgressIndicator(strokeWidth: 2),
+                            )
+                          : const Icon(Icons.my_location, size: 18),
                       label: Text(t.market.useLocation),
-                      onPressed: _useLocation,
+                      onPressed: _locationBusy ? null : _useLocation,
+                    ),
+                  ),
+                if (_locationError != null)
+                  Padding(
+                    padding: const EdgeInsets.only(top: 4),
+                    child: Text(
+                      _locationError!,
+                      style: const TextStyle(color: Color(0xFFB3261E), fontSize: 12),
                     ),
                   ),
                 const SizedBox(height: 8),
@@ -541,6 +563,7 @@ class _ConfigSheetState extends State<_ConfigSheet> {
                 ),
               ],
             ),
+          ),
     );
   }
 }