Add support for downloading media

Fixes #85

lib/helpers/file_dialog_helper.dart

@@ -0,0 +1,93 @@
+import "dart:io";
+
+import "package:file_selector/file_selector.dart";
+import "package:flutter_file_dialog/flutter_file_dialog.dart";
+import "package:flutter_riverpod/flutter_riverpod.dart";
+import "package:mime/mime.dart";
+import "package:nexus/controllers/image_picker.dart";
+import "package:nexus/controllers/portal.dart";
+import "package:xdg_desktop_portal/xdg_desktop_portal.dart";
+
+class FileDialogHelper(final Ref ref) {
+  Future<bool> saveFile(File file, String filename) async {
+    final mimeType = lookupMimeType(
+      file.path,
+      headerBytes: await file
+          .openRead(0, defaultMagicNumbersMaxLength)
+          .expand((bytes) => bytes)
+          .toList(),
+    );
+
+    final filenameWithExtension = mimeType == null
+        ? filename
+        : "$filename.${extensionFromMime(mimeType)}";
+
+    if (Platform.isAndroid || Platform.isIOS) {
+      return await FlutterFileDialog.saveFile(
+            params: .new(
+              sourceFilePath: file.path,
+              fileName: filenameWithExtension,
+            ),
+          ) !=
+          null;
+    }
+
+    final destination = Platform.isLinux
+        ? await getLinuxSaveLocation(filenameWithExtension)
+        : (await getSaveLocation(suggestedName: filenameWithExtension))?.path;
+    if (destination == null) return false;
+
+    await file.copy(destination);
+    return true;
+  }
+
+  Future<String?> getLinuxSaveLocation(String filename) async {
+    final portal = await ref.read(PortalController.provider.future);
+
+    try {
+      final result = await portal.fileChooser
+          .saveFile(title: "Save file", currentName: filename)
+          .first;
+
+      return switch (result.uris.firstOrNull) {
+        final uri? => Uri.parse(uri).toFilePath(),
+        null => null,
+      };
+    } on XdgPortalRequestCancelledException catch (_) {
+      return null;
+    }
+  }
+
+  /// Lets the user pick a file, or only images if [images] is true. Returns
+  /// null if the user cancelled.
+  Future<XFile?> pickFile({bool images = false}) async {
+    if (!Platform.isLinux) {
+      return images
+          ? ref.read(ImagePickerController.provider).pickImage(source: .gallery)
+          : openFile();
+    }
+
+    final portal = await ref.read(PortalController.provider.future);
+
+    try {
+      final result = await portal.fileChooser
+          .openFile(
+            title: images ? "Pick image" : "Pick file",
+            filters: [
+              if (images)
+                .new("Images", [XdgFileChooserMimeTypePattern("image/*")]),
+            ],
+          )
+          .first;
+
+      return switch (result.uris.firstOrNull) {
+        final uri? => XFile(Uri.parse(uri).toFilePath()),
+        null => null,
+      };
+    } on XdgPortalRequestCancelledException catch (_) {
+      return null;
+    }
+  }
+
+  static final provider = Provider<FileDialogHelper>(FileDialogHelper.new);
+}