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/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');
+}