feat(headlines-feed): add saved filters bar to FeedSliverAppBar

- Integrate SavedFiltersBar into FeedSliverAppBar as a bottom preferred size widget
- Adjust HeadlinesFeedPage to accommodate the new layout
- Update FeedSliverAppBar to be unpinned and implement PreferredSizeWidget

Author: fulleni

lib/headlines-feed/view/headlines_feed_page.dart

@@ -188,11 +188,15 @@ class _HeadlinesFeedPageState extends State<HeadlinesFeedPage> {
               child: CustomScrollView(
                 controller: _scrollController,
                 slivers: [
-                  const FeedSliverAppBar(),
-                  const SliverToBoxAdapter(
-                    child: Padding(
-                      padding: EdgeInsets.symmetric(horizontal: AppSpacing.md),
-                      child: SavedFiltersBar(),
+                  const FeedSliverAppBar(
+                    bottom: PreferredSize(
+                      preferredSize: Size.fromHeight(52),
+                      child: Padding(
+                        padding: EdgeInsets.symmetric(
+                          horizontal: AppSpacing.md,
+                        ),
+                        child: SavedFiltersBar(),
+                      ),
                     ),
                   ),
                   SliverPadding(

lib/headlines-feed/widgets/feed_sliver_app_bar.dart

@@ -18,9 +18,12 @@ import 'package:ui_kit/ui_kit.dart';
 /// [HeadlineSearchDelegate] and includes a user avatar that opens the
 /// [AccountSheet].
 /// {@endtemplate}
-class FeedSliverAppBar extends StatelessWidget {
+class FeedSliverAppBar extends StatelessWidget implements PreferredSizeWidget {
   /// {@macro feed_sliver_app_bar}
-  const FeedSliverAppBar({super.key});
+  const FeedSliverAppBar({required this.bottom, super.key});
+
+  /// This widget appears across the bottom of the app bar.
+  final PreferredSizeWidget bottom;
 
   @override
   Widget build(BuildContext context) {
@@ -29,13 +32,14 @@ class FeedSliverAppBar extends StatelessWidget {
 
     return SliverAppBar(
       // The app bar will remain visible at the top when scrolling.
-      pinned: true,
+      pinned: false,
       // The app bar will become visible as soon as the user scrolls up.
       floating: true,
       // The app bar will snap into view when scrolling up, even if the user
       // stops scrolling partway. This works in conjunction with `floating` to
       // create the desired scrolling effect.
       snap: true,
+      bottom: bottom,
       // The title is a custom search bar widget.
       title: GestureDetector(
         onTap: () {
@@ -90,4 +94,8 @@ class FeedSliverAppBar extends StatelessWidget {
       ),
     );
   }
+
+  @override
+  Size get preferredSize =>
+      Size.fromHeight(kToolbarHeight + (bottom.preferredSize.height));
 }