feat(chat): Add folder support in new chat screen

lib/core/providers/app_providers.dart

@@ -853,6 +853,21 @@ class SelectedModel extends _$SelectedModel {
   void clear() => state = null;
 }
 
+/// Tracks a pending folder ID for the next new conversation.
+///
+/// When a user starts a new chat from within a folder context menu,
+/// this provider holds the folder ID so that the conversation is
+/// automatically placed in that folder upon creation.
+@Riverpod(keepAlive: true)
+class PendingFolderId extends _$PendingFolderId {
+  @override
+  String? build() => null;
+
+  void set(String? folderId) => state = folderId;
+
+  void clear() => state = null;
+}
+
 // Track if the current model selection is manual (user-selected) or automatic (default)
 @Riverpod(keepAlive: true)
 class IsManualModelSelection extends _$IsManualModelSelection {

lib/core/services/api_service.dart

@@ -812,6 +812,7 @@ class ApiService {
     required List<ChatMessage> messages,
     String? model,
     String? systemPrompt,
+    String? folderId,
   }) async {
     _traceApi('Creating new conversation on OpenWebUI server');
     _traceApi('Title: $title, Messages: ${messages.length}');
@@ -893,7 +894,7 @@ class ApiService {
         'tags': [],
         'timestamp': DateTime.now().millisecondsSinceEpoch,
       },
-      'folder_id': null,
+      'folder_id': folderId,
     };
 
     _traceApi('Sending chat data with proper parent-child structure');