fix(mobile): drop double home-indicator padding under chat composer

chat.tsx wrote SafeAreaView edges={["top","bottom"]} while the parent
<Tabs> container already absorbs the home-indicator inset on behalf of
all tab screens. The result was ~34pt of empty space below the
composer. Sibling tabs (inbox / my-issues / more) all use
edges={["top"]} — chat was the outlier.

The gap only became visible after the floating-card composer landed;
the previous sticky-bar layout disguised it as bg-coloured padding.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/mobile/app/(app)/[workspace]/(tabs)/chat.tsx

@@ -345,10 +345,7 @@ export default function ChatTab() {
         : undefined;
 
   return (
-    <SafeAreaView
-      className="flex-1 bg-background"
-      edges={["top", "bottom"]}
-    >
+    <SafeAreaView className="flex-1 bg-background" edges={["top"]}>
       <ChatHeader
         currentSession={activeSession}
         currentAgent={currentAgent}