Add support for downloading media

Fixes #85

lib/helpers/extensions/save_attachment.dart

@@ -0,0 +1,34 @@
+import "package:flutter_riverpod/flutter_riverpod.dart";
+import "package:material_ui/material_ui.dart";
+import "package:nexus/controllers/client.dart";
+import "package:nexus/helpers/extensions/show_error_snack_bar.dart";
+import "package:nexus/helpers/extensions/with_progress_snackbar.dart";
+import "package:nexus/helpers/file_dialog_helper.dart";
+
+extension SaveAttachment on BuildContext {
+  /// Downloads the media at [mxc], displaying a snackbar whilst it's donwnloading,
+  /// then lets the user pick where to save it, suggesting [filename].
+  Future<void> saveAttachment(
+    WidgetRef ref,
+    Uri mxc, {
+    required bool encrypted,
+    String? filename,
+  }) async {
+    final file = await withProgressSnackbar(
+      "Downloading file...",
+      () => ref
+          .read(ClientController.provider.notifier)
+          .downloadMedia(.new(mxc: mxc, encrypted: encrypted)),
+    );
+    if (file == null || !mounted) return;
+
+    final messenger = ScaffoldMessenger.of(this);
+    try {
+      await ref
+          .read(FileDialogHelper.provider)
+          .saveFile(file, filename ?? mxc.pathSegments.last);
+    } catch (error) {
+      messenger.showErrorSnackBar(error.toString());
+    }
+  }
+}