autocomplete: Use directional alignment for autocomplete results

Use AlignmentDirectional.bottomStart instead of Alignment.bottomLeft
for the autocomplete dropdown positioning. This ensures the dropdown
alignment respects text direction in RTL layouts.

Fixes the issue described here:
zulip#1991 (comment)

Author: yash-agarwa-l

lib/widgets/autocomplete.dart

@@ -132,7 +132,7 @@ class _AutocompleteFieldState<QueryT extends AutocompleteQuery, ResultT extends
       //   AutocompleteView.
       optionsViewBuilder: (context, _, _) {
         return Align(
-          alignment: Alignment.bottomLeft,
+          alignment: AlignmentDirectional.bottomStart,
           child: Material(
             elevation: 4.0,
             child: ConstrainedBox(