feat(ux): edge fades hint that filter rows scroll horizontally

Chips clipped at the viewport edge with no cue that more filters exist.
New reusable EdgeFade widget (ShaderMask + dstIn) fades an edge only
while content remains beyond it, driven by scroll metrics; RTL mirrors
via Directionality. Applied to the inventory filter bar, market filter
bar and calendar month strip.

apps/app_seeds/lib/ui/edge_fade.dart

@@ -0,0 +1,82 @@
+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.
+///
+/// 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
+/// 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});
+
+  /// The horizontal scrollable to mask.
+  final Widget child;
+
+  /// Width of each faded edge in logical pixels.
+  final double fadeWidth;
+
+  @override
+  State<EdgeFade> createState() => EdgeFadeState();
+}
+
+class EdgeFadeState extends State<EdgeFade> {
+  bool _fadeStart = false;
+  bool _fadeEnd = false;
+
+  /// Whether the physical left edge is currently faded.
+  @visibleForTesting
+  bool get fadeLeft => _isRtl ? _fadeEnd : _fadeStart;
+
+  /// Whether the physical right edge is currently faded.
+  @visibleForTesting
+  bool get fadeRight => _isRtl ? _fadeStart : _fadeEnd;
+
+  bool get _isRtl => Directionality.of(context) == TextDirection.rtl;
+
+  bool _onMetrics(ScrollMetrics metrics) {
+    if (metrics.axis != Axis.horizontal) return false;
+    final start = metrics.extentBefore > 0.5;
+    final end = metrics.extentAfter > 0.5;
+    if (start != _fadeStart || end != _fadeEnd) {
+      setState(() {
+        _fadeStart = start;
+        _fadeEnd = end;
+      });
+    }
+    return false;
+  }
+
+  @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.
+    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,
+              ],
+              stops: [0, w, 1 - w, 1],
+            ).createShader(bounds);
+          },
+          child: widget.child,
+        ),
+      ),
+    );
+  }
+}