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

Author: lakshya1goel

lib/widgets/compose_box.dart

@@ -22,6 +22,63 @@ import 'store.dart';
 import 'text.dart';
 import 'theme.dart';
 
+/// Compose-box styles that differ between light and dark theme.
+///
+/// These styles will animate on theme changes (with help from [lerp]).
+class ComposeBoxTheme extends ThemeExtension<ComposeBoxTheme> {
+  factory ComposeBoxTheme.light(BuildContext context) {
+    return ComposeBoxTheme._(
+      boxShadow: null,
+    );
+  }
+
+  factory ComposeBoxTheme.dark(BuildContext context) {
+    return ComposeBoxTheme._(
+      boxShadow: [BoxShadow(
+        color: DesignVariables.dark.bgTopBar,
+        offset: const Offset(0, -4),
+        blurRadius: 16,
+        spreadRadius: 0,
+      )],
+    );
+  }
+
+  ComposeBoxTheme._({
+    required this.boxShadow,
+  });
+
+  /// The [ComposeBoxTheme] from the context's active theme.
+  ///
+  /// The [ThemeData] must include [ComposeBoxTheme] in [ThemeData.extensions].
+  static ComposeBoxTheme of(BuildContext context) {
+    final theme = Theme.of(context);
+    final extension = theme.extension<ComposeBoxTheme>();
+    assert(extension != null);
+    return extension!;
+  }
+
+  final List<BoxShadow>? boxShadow;
+
+  @override
+  ComposeBoxTheme copyWith({
+    List<BoxShadow>? boxShadow,
+  }) {
+    return ComposeBoxTheme._(
+      boxShadow: boxShadow ?? this.boxShadow,
+    );
+  }
+
+  @override
+  ComposeBoxTheme lerp(ComposeBoxTheme other, double t) {
+    if (identical(this, other)) {
+      return this;
+    }
+    return ComposeBoxTheme._(
+      boxShadow: BoxShadow.lerpList(boxShadow, other.boxShadow, t)!,
+    );
+  }
+}
+
 const double _composeButtonSize = 44;
 
 /// A [TextEditingController] for use in the compose box.
@@ -1056,7 +1113,9 @@ 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.borderBar)),
+        boxShadow: ComposeBoxTheme.of(context).boxShadow,
+      ),
       // TODO(#720) try a Stack for the overlaid linear progress indicator
       child: Material(
         color: designVariables.composeBoxBg,

lib/widgets/theme.dart

@@ -1,6 +1,7 @@
 import 'package:flutter/material.dart';
 
 import '../api/model/model.dart';
+import 'compose_box.dart';
 import 'content.dart';
 import 'emoji_reaction.dart';
 import 'message_list.dart';
@@ -32,6 +33,7 @@ ThemeData zulipThemeData(BuildContext context) {
         designVariables,
         EmojiReactionTheme.light(),
         MessageListTheme.light(),
+        ComposeBoxTheme.light(context),
       ];
     }
     case Brightness.dark: {
@@ -41,6 +43,7 @@ ThemeData zulipThemeData(BuildContext context) {
         designVariables,
         EmojiReactionTheme.dark(),
         MessageListTheme.dark(),
+        ComposeBoxTheme.dark(context),
       ];
     }
   }
@@ -175,7 +178,7 @@ class DesignVariables extends ThemeExtension<DesignVariables> {
     bgMenuButtonActive: Colors.black.withValues(alpha: 0.2),
     bgMenuButtonSelected: Colors.black.withValues(alpha: 0.25),
     bgTopBar: const Color(0xff242424),
-    borderBar: Colors.black.withValues(alpha: 0.5),
+    borderBar: const Color(0xffffffff).withValues(alpha: 0.1),
     borderMenuButtonSelected: Colors.white.withValues(alpha: 0.1),
     btnLabelAttLowIntDanger: const Color(0xffff8b7c),
     btnLabelAttMediumIntDanger: const Color(0xffff8b7c),

pubspec.lock

@@ -138,10 +138,10 @@ packages:
     dependency: transitive
     description:
       name: characters
-      sha256: "04a925763edad70e8443c99234dc3328f442e811f1d8fd1a72f1c8ad0f69a605"
+      sha256: f71061c654a3380576a52b451dd5532377954cf9dbd272a78fc8479606670803
       url: "https://pub.dev"
     source: hosted
-    version: "1.3.0"
+    version: "1.4.0"
   charcode:
     dependency: transitive
     description:
@@ -194,10 +194,10 @@ packages:
     dependency: "direct main"
     description:
       name: collection
-      sha256: a1ace0a119f20aabc852d165077c036cd864315bd99b7eaa10a60100341941bf
+      sha256: "2f5709ae4d3d59dd8f7cd309b4e023046b57d8a6c82130785d2b0e5868084e76"
       url: "https://pub.dev"
     source: hosted
-    version: "1.19.0"
+    version: "1.19.1"
   color_models:
     dependency: "direct overridden"
     description:
@@ -711,10 +711,10 @@ packages:
     dependency: transitive
     description:
       name: matcher
-      sha256: d2323aa2060500f906aa31a895b4030b6da3ebdcc5619d14ce1aada65cd161cb
+      sha256: dc58c723c3c24bf8d3e2d3ad3f2f9d7bd9cf43ec6feaa64181775e60190153f2
       url: "https://pub.dev"
     source: hosted
-    version: "0.12.16+1"
+    version: "0.12.17"
   material_color_utilities:
     dependency: transitive
     description:
@@ -727,10 +727,10 @@ packages:
     dependency: transitive
     description:
       name: meta
-      sha256: bdb68674043280c3428e9ec998512fb681678676b3c54e773629ffe74419f8c7
+      sha256: e3641ec5d63ebf0d9b41bd43201a66e3fc79a65db5f61fc181f04cd27aab950c
       url: "https://pub.dev"
     source: hosted
-    version: "1.15.0"
+    version: "1.16.0"
   mime:
     dependency: "direct main"
     description:
@@ -1044,18 +1044,18 @@ packages:
     dependency: "direct dev"
     description:
       name: stack_trace
-      sha256: "9f47fd3630d76be3ab26f0ee06d213679aa425996925ff3feffdec504931c377"
+      sha256: "8b27215b45d22309b5cddda1aa2b19bdfec9df0e765f2de506401c071d38d1b1"
       url: "https://pub.dev"
     source: hosted
-    version: "1.12.0"
+    version: "1.12.1"
   stream_channel:
     dependency: transitive
     description:
       name: stream_channel
-      sha256: ba2aa5d8cc609d96bbb2899c28934f9e1af5cddbd60a827822ea467161eb54e7
+      sha256: "969e04c80b8bcdf826f8f16579c7b14d780458bd97f56d107d3950fdbeef059d"
       url: "https://pub.dev"
     source: hosted
-    version: "2.1.2"
+    version: "2.1.4"
   stream_transform:
     dependency: transitive
     description:
@@ -1068,10 +1068,10 @@ packages:
     dependency: transitive
     description:
       name: string_scanner
-      sha256: "0bd04f5bb74fcd6ff0606a888a30e917af9bd52820b178eaa464beb11dca84b6"
+      sha256: "921cd31725b72fe181906c6a94d987c78e3b98c2e205b397ea399d4054872b43"
       url: "https://pub.dev"
     source: hosted
-    version: "1.4.0"
+    version: "1.4.1"
   sync_http:
     dependency: transitive
     description:
@@ -1084,10 +1084,10 @@ packages:
     dependency: transitive
     description:
       name: term_glyph
-      sha256: a29248a84fbb7c79282b40b8c72a1209db169a2e0542bce341da992fe1bc7e84
+      sha256: "7f554798625ea768a7518313e58f83891c7f5024f88e46e7182a4558850a4b8e"
       url: "https://pub.dev"
     source: hosted
-    version: "1.2.1"
+    version: "1.2.2"
   test:
     dependency: "direct dev"
     description: