fix: use local app data directory on desktop platforms

lib/core/infrastructure/platform_file_system_service.dart

@@ -132,7 +132,18 @@ class PlatformFileSystemService implements FileSystemService {
     // The actual tracking is done in ProtectedFilesRegistry.registerExternalPath()
   }
 
-  /// Gets the app documents directory with caching
+  /// Gets the model storage directory with caching.
+  ///
+  /// On desktop platforms, uses platform-specific local app data directories
+  /// that are NOT synced to cloud services (OneDrive, iCloud, Dropbox).
+  /// This is critical because native code (LiteRT-LM JNI) cannot reliably
+  /// access files in cloud-synced folders.
+  ///
+  /// Storage locations:
+  /// - Windows: %LOCALAPPDATA%\flutter_gemma (truly local, never synced)
+  /// - macOS: ~/Library/Application Support/flutter_gemma
+  /// - Linux: ~/.local/share/flutter_gemma
+  /// - Android/iOS: App documents directory (standard behavior)
   Future<Directory> _getDocumentsDirectory() async {
     // Web doesn't support local file system
     if (kIsWeb) {
@@ -143,7 +154,40 @@ class PlatformFileSystemService implements FileSystemService {
       return _documentsDirectory!;
     }
 
-    _documentsDirectory = await getApplicationDocumentsDirectory();
+    if (Platform.isWindows) {
+      // Use LOCALAPPDATA on Windows - truly local and never synced
+      final localAppData = Platform.environment['LOCALAPPDATA'];
+      if (localAppData != null) {
+        _documentsDirectory = Directory(path.join(localAppData, 'OroForge', 'models'));
+      } else {
+        _documentsDirectory = await getApplicationSupportDirectory();
+      }
+    } else if (Platform.isMacOS) {
+      // Use Application Support on macOS - not synced by default
+      final home = Platform.environment['HOME'];
+      if (home != null) {
+        _documentsDirectory = Directory(path.join(home, 'Library', 'Application Support', 'OroForge', 'models'));
+      } else {
+        _documentsDirectory = await getApplicationSupportDirectory();
+      }
+    } else if (Platform.isLinux) {
+      // Use XDG data directory on Linux
+      final xdgDataHome = Platform.environment['XDG_DATA_HOME'];
+      if (xdgDataHome != null) {
+        _documentsDirectory = Directory(path.join(xdgDataHome, 'OroForge', 'models'));
+      } else {
+        final home = Platform.environment['HOME'];
+        if (home != null) {
+          _documentsDirectory = Directory(path.join(home, '.local', 'share', 'OroForge', 'models'));
+        } else {
+          _documentsDirectory = await getApplicationSupportDirectory();
+        }
+      }
+    } else {
+      // Mobile platforms use standard documents directory
+      _documentsDirectory = await getApplicationDocumentsDirectory();
+    }
+
     return _documentsDirectory!;
   }
 }

lib/core/model_management/utils/file_system_manager.dart

@@ -76,9 +76,71 @@ class ModelFileSystemManager {
     }
   }
 
+  /// Cached model storage directory
+  static Directory? _modelStorageDirectory;
+
+  /// Gets the model storage directory.
+  ///
+  /// On desktop platforms, uses local app data directories that are NOT synced
+  /// to cloud services (OneDrive, iCloud, Dropbox). This is critical because
+  /// native code (LiteRT-LM JNI) cannot reliably access files in cloud-synced
+  /// folders.
+  ///
+  /// Storage locations:
+  /// - Windows: %LOCALAPPDATA%\flutter_gemma (truly local, never synced)
+  /// - macOS: ~/Library/Application Support/flutter_gemma
+  /// - Linux: ~/.local/share/flutter_gemma
+  /// - Android/iOS: App documents directory (standard behavior)
+  static Future<Directory> getModelStorageDirectory() async {
+    if (_modelStorageDirectory != null) {
+      return _modelStorageDirectory!;
+    }
+
+    if (Platform.isWindows) {
+      // Use LOCALAPPDATA on Windows - truly local and never synced
+      final localAppData = Platform.environment['LOCALAPPDATA'];
+      if (localAppData != null) {
+        _modelStorageDirectory = Directory(path.join(localAppData, 'OroForge', 'models'));
+      } else {
+        _modelStorageDirectory = await getApplicationSupportDirectory();
+      }
+    } else if (Platform.isMacOS) {
+      // Use Application Support on macOS - not synced by default
+      final localAppData = Platform.environment['HOME'];
+      if (localAppData != null) {
+        _modelStorageDirectory = Directory(path.join(localAppData, 'Library', 'Application Support', 'OroForge', 'models'));
+      } else {
+        _modelStorageDirectory = await getApplicationSupportDirectory();
+      }
+    } else if (Platform.isLinux) {
+      // Use XDG data directory on Linux
+      final xdgDataHome = Platform.environment['XDG_DATA_HOME'];
+      if (xdgDataHome != null) {
+        _modelStorageDirectory = Directory(path.join(xdgDataHome, 'OroForge', 'models'));
+      } else {
+        final home = Platform.environment['HOME'];
+        if (home != null) {
+          _modelStorageDirectory = Directory(path.join(home, '.local', 'share', 'OroForge', 'models'));
+        } else {
+          _modelStorageDirectory = await getApplicationSupportDirectory();
+        }
+      }
+    } else {
+      // Mobile platforms use standard documents directory
+      _modelStorageDirectory = await getApplicationDocumentsDirectory();
+    }
+
+    // Ensure directory exists
+    if (!await _modelStorageDirectory!.exists()) {
+      await _modelStorageDirectory!.create(recursive: true);
+    }
+
+    return _modelStorageDirectory!;
+  }
+
   /// Gets the full file path for a model file with Android path correction
   static Future<String> getModelFilePath(String filename) async {
-    final directory = await getApplicationDocumentsDirectory();
+    final directory = await getModelStorageDirectory();
     return getCorrectedPath(directory.path, filename);
   }
 
@@ -90,7 +152,7 @@ class ModelFileSystemManager {
     List<String>? protectedFiles,
     List<String>? supportedExtensions,
   }) async {
-    final directory = await getApplicationDocumentsDirectory();
+    final directory = await getModelStorageDirectory();
     final extensions = supportedExtensions ?? ModelFileSystemManager.supportedExtensions;
     final protected = protectedFiles ?? [];
 
@@ -132,7 +194,7 @@ class ModelFileSystemManager {
   static Future<StorageStats> getStorageInfo({
     List<String>? protectedFiles,
   }) async {
-    final directory = await getApplicationDocumentsDirectory();
+    final directory = await getModelStorageDirectory();
 
     final files = directory
         .listSync()

lib/mobile/smart_downloader.dart

@@ -1,9 +1,11 @@
 import 'dart:async';
+import 'dart:io' show Platform;
 import 'package:flutter/foundation.dart';
 import 'package:background_downloader/background_downloader.dart';
 import 'package:flutter_gemma/core/domain/download_error.dart';
 import 'package:flutter_gemma/core/domain/download_exception.dart';
 import 'package:flutter_gemma/core/model_management/cancel_token.dart';
+import 'package:path/path.dart' as p;
 
 /// Smart downloader with HTTP-aware retry logic
 ///
@@ -177,7 +179,28 @@ class SmartDownloader {
     StreamSubscription? listener;
 
     try {
-      final (baseDirectory, directory, filename) = await Task.split(filePath: targetPath);
+      // CRITICAL: Task.split() has a bug on Windows where it strips the drive letter
+      // from paths that don't match known base directories, causing files to be written
+      // to the wrong location. We handle Windows paths manually to preserve the full path.
+      final BaseDirectory baseDirectory;
+      final String directory;
+      final String filename;
+
+      if (!kIsWeb && Platform.isWindows) {
+        // On Windows, use BaseDirectory.root with the FULL directory path including drive letter.
+        // background_downloader's baseDirectoryPath(BaseDirectory.root) returns '' on Windows,
+        // so path.join('', fullDirectory, filename) correctly produces the absolute path.
+        baseDirectory = BaseDirectory.root;
+        directory = p.dirname(targetPath);  // Full directory with drive letter (e.g., C:\Users\...)
+        filename = p.basename(targetPath);
+        debugPrint('🔵 Windows path handling: dir=$directory, file=$filename');
+      } else {
+        // On other platforms, Task.split() works correctly
+        final splitResult = await Task.split(filePath: targetPath);
+        baseDirectory = splitResult.$1;
+        directory = splitResult.$2;
+        filename = splitResult.$3;
+      }
 
       final task = DownloadTask(
         url: url,