subscription_list [nfc]: Handle bottom insets explicitly in subscription list page

This reverts part of 742320c for SubscriptionListPageBody,
as this widget is planned to be used outside the context of home
page, specifically for the upcoming share page.

Author: rajveermalviya

lib/widgets/subscription_list.dart

@@ -119,7 +119,18 @@ class _SubscriptionListPageBodyState extends State<SubscriptionListPageBody> wit
         message: zulipLocalizations.channelsEmptyPlaceholder);
     }
 
-    return SafeArea( // horizontal insets
+    return SafeArea(
+      // Don't pad the bottom here; we want the list content to do that.
+      //
+      // When this page is used in the context of the home page, this
+      // param and the below use of `SliverSafeArea` would be noop, because
+      // `Scaffold.bottomNavigationBar` in the home page handles that for us.
+      // But this page is planned to be used for share-to-zulip page, so we
+      // need this to be handled here.
+      //
+      // Other *PageBody widgets don't handle this because they aren't
+      // planned to be (re-)used outside the context of the home page.
+      bottom: false,
       child: CustomScrollView(
         slivers: [
           if (pinned.isNotEmpty) ...[
@@ -140,6 +151,9 @@ class _SubscriptionListPageBodyState extends State<SubscriptionListPageBody> wit
           ],
 
           // TODO(#188): add button leading to "All Streams" page with ability to subscribe
+
+          // This ensures last item in scrollable can settle in an unobstructed area.
+          const SliverSafeArea(sliver: SliverToBoxAdapter(child: SizedBox.shrink())),
         ]));
   }
 }