♻️ Refactored all ScaffoldMessager to use unifined snackbar API

lib/widgets/post/compose_embed_sheet.dart

@@ -6,6 +6,7 @@ import 'package:flutter_hooks/flutter_hooks.dart';
 import 'package:gap/gap.dart';
 import 'package:hooks_riverpod/hooks_riverpod.dart';
 import 'package:island/models/post.dart';
+import 'package:island/widgets/alert.dart';
 import 'package:island/widgets/content/sheet.dart';
 import 'package:island/widgets/post/compose_shared.dart';
 import 'package:material_symbols_icons/symbols.dart';
@@ -56,9 +57,7 @@ class ComposeEmbedSheet extends HookConsumerWidget {
     void saveEmbedView() {
       final uri = uriController.text.trim();
       if (uri.isEmpty) {
-        ScaffoldMessenger.of(
-          context,
-        ).showSnackBar(SnackBar(content: Text('embedUriRequired'.tr())));
+        showSnackBar('embedUriRequired'.tr());
         return;
       }
 

lib/widgets/post/compose_shared.dart

@@ -751,12 +751,7 @@ class ComposeLogic {
 
       return post;
     } catch (err) {
-      // Show error message if context is mounted
-      if (context.mounted) {
-        ScaffoldMessenger.of(
-          context,
-        ).showSnackBar(SnackBar(content: Text('Error: $err')));
-      }
+      showErrorAlert(err);
       rethrow;
     } finally {
       state.submitting.value = false;