feat: import Chinese-localized Buzz source snapshot

Signed-off-by: cls_宁波本机 <908705107@qq.com>

mobile/lib/shared/widgets/mobile_tab_footer_backdrop.dart

@@ -0,0 +1,86 @@
+import 'package:flutter/material.dart';
+
+import '../theme/theme.dart';
+
+/// Height of the floating mobile tab bar, excluding its bottom clearance.
+const mobileTabBarHeight = 56.0;
+
+/// Gap between the floating mobile tab bar and the bottom safe area.
+const mobileTabBarBottomGap = Grid.twelve;
+
+/// Fixed visual height of the shared footer fade behind the floating tab bar.
+double mobileTabFooterBackdropHeight(BuildContext _) => 180;
+
+/// Builds the shared transparent-to-surface footer fade.
+///
+/// Kept separate from [MobileTabFooterBackdrop] so floating controls such as
+/// the channel composer can paint the exact same fade behind their own content.
+LinearGradient mobileTabFooterBackdropGradient(
+  BuildContext context, {
+  List<double> stops = const [0, 0.18, 0.38, 0.6, 0.8, 1],
+  List<double> opacities = const [0, 0.03, 0.12, 0.34, 0.7, 1],
+  Color? tint,
+  double tintBlend = 0,
+}) {
+  assert(stops.length == opacities.length);
+  assert(tintBlend >= 0 && tintBlend <= 1);
+  final surface = Color.lerp(context.colors.surface, tint, tintBlend)!;
+  return LinearGradient(
+    begin: Alignment.topCenter,
+    end: Alignment.bottomCenter,
+    stops: stops,
+    colors: [
+      for (final opacity in opacities) surface.withValues(alpha: opacity),
+    ],
+  );
+}
+
+/// Shared fade behind the floating mobile tab bar.
+class MobileTabFooterBackdrop extends StatelessWidget {
+  /// Vertical extent of the backdrop in logical pixels.
+  final double height;
+
+  /// Gradient stop positions, from the transparent top to the opaque bottom.
+  final List<double> stops;
+
+  /// Surface-color alpha values paired with [stops].
+  final List<double> opacities;
+
+  /// Optional color blended into the surface before opacity is applied.
+  final Color? tint;
+
+  /// Amount of [tint] mixed into the surface, from 0 to 1.
+  final double tintBlend;
+
+  /// Creates a footer backdrop with the required [height].
+  ///
+  /// Override [stops] and [opacities] together to customize the gradient.
+  const MobileTabFooterBackdrop({
+    super.key,
+    required this.height,
+    this.stops = const [0, 0.18, 0.38, 0.6, 0.8, 1],
+    this.opacities = const [0, 0.03, 0.12, 0.34, 0.7, 1],
+    this.tint,
+    this.tintBlend = 0,
+  }) : assert(stops.length == opacities.length),
+       assert(tintBlend >= 0 && tintBlend <= 1);
+
+  @override
+  Widget build(BuildContext context) {
+    return SizedBox(
+      height: height,
+      width: double.infinity,
+      child: DecoratedBox(
+        decoration: BoxDecoration(
+          gradient: mobileTabFooterBackdropGradient(
+            context,
+            stops: stops,
+            opacities: opacities,
+            tint: tint,
+            tintBlend: tintBlend,
+          ),
+        ),
+      ),
+    );
+  }
+}