iOS Share Extension: pick the destination inside the share sheet

SwiftUI folder picker in the extension (WebDAV PROPFIND), account shared via
a Keychain group, upload on a background session that outlives the extension
and is finished by the app. The inbox + notification hand-off remains as the
fallback (no account / 'choose later'). Shared code moves to ios/Shared.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

lib/main.dart

@@ -22,6 +22,7 @@ import 'providers/shares_controller.dart';
 import 'providers/sync_status_controller.dart';
 import 'providers/trash_controller.dart';
 import 'services/pick_intent_service.dart';
+import 'services/share_account_service.dart';
 import 'services/share_intent_service.dart';
 import 'theme/app_theme.dart';
 import 'theme/design_tokens.dart';
@@ -58,7 +59,16 @@ void main() {
       providers: [
         ChangeNotifierProvider(create: (_) => ConnectivityController()),
         ChangeNotifierProvider(
-          create: (context) => SessionController(context.read()),
+          create: (context) {
+            final session = SessionController(context.read());
+            // iOS's Share Extension has no Flutter engine; it picks its
+            // folders/uploads as whichever account was last published here.
+            session.addAccountReadyListener(
+              () => ShareAccountService.publish(session),
+            );
+            session.addAccountClearedListener(ShareAccountService.clear);
+            return session;
+          },
         ),
         ChangeNotifierProvider(create: (_) => SettingsController()),
         // Before SyncStatusController, which follows its Files Cache rule

lib/services/share_account_service.dart

@@ -0,0 +1,30 @@
+import 'package:flutter/services.dart';
+import '../providers/session_controller.dart';
+import 'native_channel.dart';
+
+/// Tells the native side which account is active, so iOS's Share Extension -
+/// a separate process with no Flutter engine - can list folders and upload as
+/// it without opening the app (`ios/Runner/Native/NativeServices.swift`'s
+/// `share_account`, stored in a Keychain group shared with the extension).
+/// Android has no equivalent (its share intent opens the app itself), so
+/// there the channel simply doesn't exist and both calls are no-ops.
+class ShareAccountService {
+  static const _channel = MethodChannel('dev.ayushya.noo/share_account');
+
+  /// Publishes the active account (call whenever one becomes active).
+  static Future<void> publish(SessionController session) async {
+    final authHeader = session.service?.authHeaders['Authorization'];
+    if (authHeader == null || session.serverUrl.isEmpty) return;
+    final host = Uri.tryParse(session.serverUrl)?.host ?? session.serverUrl;
+    await invokeIfAvailable(_channel, 'setAccount', {
+      'serverUrl': session.serverUrl,
+      'username': session.username,
+      'authHeader': authHeader,
+      'displayName': '${session.username}@$host',
+    });
+  }
+
+  /// Forgets it (sign-out / switching away) so the extension stops offering
+  /// uploads to an account that's no longer signed in.
+  static Future<void> clear() => invokeIfAvailable(_channel, 'clearAccount');
+}