feat(#173): dégradés haut/bas sur la liste de bulles scrollable

Même logique que le slider enfants : fondu aux bords sauf au
premier et au dernier élément.

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/lib/widgets/quotidien/cartes_feed.dart

@@ -22,17 +22,32 @@ class CartesFeed extends StatefulWidget {
 }
 
 class _CartesFeedState extends State<CartesFeed> {
+  static const double _fadeExtent = 0.08;
+
   List<CarteBulle> _items = const [];
   bool _loading = true;
   String? _error;
   String? _busyCardId;
 
+  final ScrollController _scrollController = ScrollController();
+  bool _isScrollable = false;
+  bool _fadeTop = false;
+  bool _fadeBottom = false;
+
   @override
   void initState() {
     super.initState();
+    _scrollController.addListener(_syncScrollFades);
     _load();
   }
 
+  @override
+  void dispose() {
+    _scrollController.removeListener(_syncScrollFades);
+    _scrollController.dispose();
+    super.dispose();
+  }
+
   @override
   void didUpdateWidget(covariant CartesFeed oldWidget) {
     super.didUpdateWidget(oldWidget);
@@ -41,6 +56,25 @@ class _CartesFeedState extends State<CartesFeed> {
     }
   }
 
+  void _syncScrollFades() {
+    if (!_scrollController.hasClients) return;
+    final position = _scrollController.position;
+    final scrollable = position.maxScrollExtent > 0.0;
+    final threshold = position.viewportDimension * _fadeExtent / 2;
+    final fadeTop = scrollable && position.pixels > threshold;
+    final fadeBottom = scrollable &&
+        position.pixels < (position.maxScrollExtent - threshold);
+    if (scrollable != _isScrollable ||
+        fadeTop != _fadeTop ||
+        fadeBottom != _fadeBottom) {
+      setState(() {
+        _isScrollable = scrollable;
+        _fadeTop = fadeTop;
+        _fadeBottom = fadeBottom;
+      });
+    }
+  }
+
   Future<void> _load() async {
     setState(() {
       _loading = true;
@@ -55,6 +89,7 @@ class _CartesFeedState extends State<CartesFeed> {
         _items = items;
         _loading = false;
       });
+      WidgetsBinding.instance.addPostFrameCallback((_) => _syncScrollFades());
     } catch (e) {
       if (!mounted) return;
       setState(() {
@@ -304,10 +339,52 @@ class _CartesFeedState extends State<CartesFeed> {
       );
     }
 
-    return RefreshIndicator(
+    return NotificationListener<ScrollMetricsNotification>(
+      onNotification: (_) {
+        _syncScrollFades();
+        return false;
+      },
+      child: ShaderMask(
+        blendMode: BlendMode.dstIn,
+        shaderCallback: (Rect bounds) {
+          final stops = <double>[
+            0.0,
+            _fadeExtent,
+            1.0 - _fadeExtent,
+            1.0,
+          ];
+          if (!_isScrollable) {
+            return const LinearGradient(
+              begin: Alignment.topCenter,
+              end: Alignment.bottomCenter,
+              colors: <Color>[
+                Colors.black,
+                Colors.black,
+                Colors.black,
+                Colors.black,
+              ],
+              stops: [0.0, 0.08, 0.92, 1.0],
+            ).createShader(bounds);
+          }
+          final topMask = _fadeTop ? Colors.transparent : Colors.black;
+          final bottomMask = _fadeBottom ? Colors.transparent : Colors.black;
+          return LinearGradient(
+            begin: Alignment.topCenter,
+            end: Alignment.bottomCenter,
+            colors: <Color>[
+              topMask,
+              Colors.black,
+              Colors.black,
+              bottomMask,
+            ],
+            stops: stops,
+          ).createShader(bounds);
+        },
+        child: RefreshIndicator(
           onRefresh: _load,
           child: ListView.separated(
-        padding: const EdgeInsets.only(bottom: 8),
+            controller: _scrollController,
+            padding: const EdgeInsets.only(top: 4, bottom: 8),
             itemCount: _items.length,
             separatorBuilder: (_, __) => const SizedBox(height: 10),
             itemBuilder: (context, index) {
@@ -321,6 +398,8 @@ class _CartesFeedState extends State<CartesFeed> {
               );
             },
           ),
+        ),
+      ),
     );
   }
 }