compose: Compose-box border is subtle in dark mode

Updated the top border and added shadow effect to the top of
the compose box to ensure it is subtle in dark mode.

For reference, see the Figma design:
https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=5908-64038&t=alSmAmdRXFDwT4IT-1

Fixes: #1207

lib/widgets/compose_box.dart

@@ -1051,7 +1051,21 @@ class _ComposeBoxContainer extends StatelessWidget {
     //   the message list itself
     return Container(width: double.infinity,
       decoration: BoxDecoration(
-        border: Border(top: BorderSide(color: designVariables.borderBar))),
+        border: Border(
+          top: BorderSide(
+            color: designVariables.composeBoxBorderBar,
+            width: 1.0,
+          ),
+        ),
+        boxShadow: [
+          BoxShadow(
+            color: designVariables.composeBoxShadow,
+            offset: const Offset(0, -4),
+            blurRadius: 16,
+            spreadRadius: 0,
+          ),
+        ],
+      ),
       // TODO(#720) try a Stack for the overlaid linear progress indicator
       child: Material(
         color: designVariables.composeBoxBg,

lib/widgets/theme.dart

@@ -132,6 +132,8 @@ class DesignVariables extends ThemeExtension<DesignVariables> {
       btnLabelAttLowIntDanger: const Color(0xffc0070a),
       btnLabelAttMediumIntDanger: const Color(0xffac0508),
       composeBoxBg: const Color(0xffffffff),
+      composeBoxBorderBar: Colors.black.withValues(alpha: 0.2),
+      composeBoxShadow: const Color(0xff242424),
       contextMenuCancelText: const Color(0xff222222),
       contextMenuItemBg: const Color(0xff6159e1),
       contextMenuItemText: const Color(0xff381da7),
@@ -182,6 +184,8 @@ class DesignVariables extends ThemeExtension<DesignVariables> {
       btnLabelAttLowIntDanger: const Color(0xffff8b7c),
       btnLabelAttMediumIntDanger: const Color(0xffff8b7c),
       composeBoxBg: const Color(0xff0f0f0f),
+      composeBoxBorderBar: Colors.white.withValues(alpha: 0.1),
+      composeBoxShadow: const Color(0xff242424),
       contextMenuCancelText: const Color(0xffffffff).withValues(alpha: 0.75),
       contextMenuItemBg: const Color(0xff7977fe),
       contextMenuItemText: const Color(0xff9398fd),
@@ -239,6 +243,8 @@ class DesignVariables extends ThemeExtension<DesignVariables> {
     required this.btnLabelAttLowIntDanger,
     required this.btnLabelAttMediumIntDanger,
     required this.composeBoxBg,
+    required this.composeBoxBorderBar,
+    required this.composeBoxShadow,
     required this.contextMenuCancelText,
     required this.contextMenuItemBg,
     required this.contextMenuItemText,
@@ -297,6 +303,8 @@ class DesignVariables extends ThemeExtension<DesignVariables> {
   final Color btnLabelAttLowIntDanger;
   final Color btnLabelAttMediumIntDanger;
   final Color composeBoxBg;
+  final Color composeBoxBorderBar;
+  final Color composeBoxShadow;
   final Color contextMenuCancelText;
   final Color contextMenuItemBg;
   final Color contextMenuItemText;
@@ -350,6 +358,8 @@ class DesignVariables extends ThemeExtension<DesignVariables> {
     Color? btnLabelAttLowIntDanger,
     Color? btnLabelAttMediumIntDanger,
     Color? composeBoxBg,
+    Color? composeBoxBorderBar,
+    Color? composeBoxShadow,
     Color? contextMenuCancelText,
     Color? contextMenuItemBg,
     Color? contextMenuItemText,
@@ -398,6 +408,8 @@ class DesignVariables extends ThemeExtension<DesignVariables> {
       btnLabelAttLowIntDanger: btnLabelAttLowIntDanger ?? this.btnLabelAttLowIntDanger,
       btnLabelAttMediumIntDanger: btnLabelAttMediumIntDanger ?? this.btnLabelAttMediumIntDanger,
       composeBoxBg: composeBoxBg ?? this.composeBoxBg,
+      composeBoxBorderBar: composeBoxBorderBar ?? this.composeBoxBorderBar,
+      composeBoxShadow: composeBoxShadow ?? this.composeBoxShadow,
       contextMenuCancelText: contextMenuCancelText ?? this.contextMenuCancelText,
       contextMenuItemBg: contextMenuItemBg ?? this.contextMenuItemBg,
       contextMenuItemText: contextMenuItemText ?? this.contextMenuItemBg,
@@ -453,6 +465,8 @@ class DesignVariables extends ThemeExtension<DesignVariables> {
       btnLabelAttLowIntDanger: Color.lerp(btnLabelAttLowIntDanger, other.btnLabelAttLowIntDanger, t)!,
       btnLabelAttMediumIntDanger: Color.lerp(btnLabelAttMediumIntDanger, other.btnLabelAttMediumIntDanger, t)!,
       composeBoxBg: Color.lerp(composeBoxBg, other.composeBoxBg, t)!,
+      composeBoxBorderBar: Color.lerp(composeBoxBorderBar, other.composeBoxBorderBar, t)!,
+      composeBoxShadow: Color.lerp(composeBoxShadow, other.composeBoxShadow, t)!,
       contextMenuCancelText: Color.lerp(contextMenuCancelText, other.contextMenuCancelText, t)!,
       contextMenuItemBg: Color.lerp(contextMenuItemBg, other.contextMenuItemBg, t)!,
       contextMenuItemText: Color.lerp(contextMenuItemText, other.contextMenuItemBg, t)!,