fix(ux): make the scroll cue visible — edge scrim + chevron

The fade-to-transparent was invisible: the row sits on the same pale
canvas, so masking the edge to transparent just revealed more of the
same colour (and often landed on an inter-chip gap). Replace it with a
surface-coloured scrim the chips slide under, topped by an outward
chevron, shown only on an edge with more content. RTL-mirrored.

apps/app_seeds/lib/ui/edge_fade.dart

@@ -1,21 +1,25 @@
 import 'package:flutter/material.dart';
 
-/// Fades the edges of a horizontal scrollable to transparency wherever more
-/// content lies beyond that edge, so it is obvious the row scrolls.
+import 'theme.dart';
+
+/// Marks the edges of a horizontal scrollable that have more content beyond
+/// them, so it is obvious the row scrolls.
 ///
 /// Wrap directly around a horizontal [SingleChildScrollView] or [ListView].
-/// The fade is a [ShaderMask] with [BlendMode.dstIn], so it melts into
-/// whatever background sits behind — no theme colour involved. Edge state is
+/// Each overflowing edge gets a soft scrim in the surface colour — the chips
+/// appear to slide *under* it — topped by a chevron pointing outward. The
+/// scrim only shows while content remains beyond that edge; it disappears once
+/// the row reaches that end (or does not overflow at all). Edge state is
 /// tracked per logical side (start/end) and mapped to physical left/right via
 /// [Directionality], so RTL mirrors correctly.
 class EdgeFade extends StatefulWidget {
-  const EdgeFade({super.key, required this.child, this.fadeWidth = 28});
+  const EdgeFade({super.key, required this.child, this.scrimWidth = 40});
 
-  /// The horizontal scrollable to mask.
+  /// The horizontal scrollable to mark.
   final Widget child;
 
-  /// Width of each faded edge in logical pixels.
-  final double fadeWidth;
+  /// Width of each edge scrim in logical pixels.
+  final double scrimWidth;
 
   @override
   State<EdgeFade> createState() => EdgeFadeState();
@@ -25,11 +29,11 @@ class EdgeFadeState extends State<EdgeFade> {
   bool _fadeStart = false;
   bool _fadeEnd = false;
 
-  /// Whether the physical left edge is currently faded.
+  /// Whether the physical left edge currently shows a scroll cue.
   @visibleForTesting
   bool get fadeLeft => _isRtl ? _fadeEnd : _fadeStart;
 
-  /// Whether the physical right edge is currently faded.
+  /// Whether the physical right edge currently shows a scroll cue.
   @visibleForTesting
   bool get fadeRight => _isRtl ? _fadeStart : _fadeEnd;
 
@@ -50,31 +54,70 @@ class EdgeFadeState extends State<EdgeFade> {
 
   @override
   Widget build(BuildContext context) {
-    final fadeLeft = this.fadeLeft;
-    final fadeRight = this.fadeRight;
-    // The mask stays in the tree even when both edges are opaque: swapping it
-    // in and out would re-inflate the child and reset its scroll position.
+    final surface = Theme.of(context).scaffoldBackgroundColor;
     return NotificationListener<ScrollMetricsNotification>(
       onNotification: (n) => n.depth == 0 ? _onMetrics(n.metrics) : false,
       child: NotificationListener<ScrollNotification>(
         onNotification: (n) => n.depth == 0 ? _onMetrics(n.metrics) : false,
-        child: ShaderMask(
-          blendMode: BlendMode.dstIn,
-          shaderCallback: (bounds) {
-            final w = bounds.width > 0
-                ? (widget.fadeWidth / bounds.width).clamp(0.0, 0.5)
-                : 0.0;
-            return LinearGradient(
-              colors: [
-                fadeLeft ? Colors.transparent : Colors.white,
-                Colors.white,
-                Colors.white,
-                fadeRight ? Colors.transparent : Colors.white,
+        child: Stack(
+          children: [
+            widget.child,
+            if (fadeLeft)
+              _EdgeCue(
+                left: true,
+                width: widget.scrimWidth,
+                surface: surface,
+              ),
+            if (fadeRight)
+              _EdgeCue(
+                left: false,
+                width: widget.scrimWidth,
+                surface: surface,
+              ),
           ],
-              stops: [0, w, 1 - w, 1],
-            ).createShader(bounds);
-          },
-          child: widget.child,
+        ),
+      ),
+    );
+  }
+}
+
+/// A gradient scrim over one physical edge of the row, opaque at the very edge
+/// and fading inward, with a chevron pointing outward toward the hidden chips.
+/// Ignores pointers so taps still reach the chip underneath.
+class _EdgeCue extends StatelessWidget {
+  const _EdgeCue({
+    required this.left,
+    required this.width,
+    required this.surface,
+  });
+
+  final bool left;
+  final double width;
+  final Color surface;
+
+  @override
+  Widget build(BuildContext context) {
+    return Positioned(
+      top: 0,
+      bottom: 0,
+      left: left ? 0 : null,
+      right: left ? null : 0,
+      width: width,
+      child: IgnorePointer(
+        child: Container(
+          alignment: left ? Alignment.centerLeft : Alignment.centerRight,
+          decoration: BoxDecoration(
+            gradient: LinearGradient(
+              begin: left ? Alignment.centerLeft : Alignment.centerRight,
+              end: left ? Alignment.centerRight : Alignment.centerLeft,
+              colors: [surface, surface.withValues(alpha: 0)],
+            ),
+          ),
+          child: Icon(
+            left ? Icons.chevron_left : Icons.chevron_right,
+            size: 22,
+            color: seedMuted,
+          ),
         ),
       ),
     );

apps/app_seeds/test/ui/edge_fade_test.dart

@@ -35,6 +35,16 @@ void main() {
     await tester.pump(); // let the post-layout metrics notification land
     expect(state(tester).fadeLeft, isFalse);
     expect(state(tester).fadeRight, isFalse);
+    expect(find.byIcon(Icons.chevron_left), findsNothing);
+    expect(find.byIcon(Icons.chevron_right), findsNothing);
+  });
+
+  testWidgets('a trailing chevron cue appears when the row overflows',
+      (tester) async {
+    await tester.pumpWidget(harness(count: 20));
+    await tester.pump();
+    expect(find.byIcon(Icons.chevron_right), findsOneWidget);
+    expect(find.byIcon(Icons.chevron_left), findsNothing);
   });
 
   testWidgets('LTR: at start only the trailing (right) edge fades',