Replace receive_sharing_intent; add background upload service; fix add-account login page

Sharing a large file to Noo used to black-screen and silently kick back
to the home screen: receive_sharing_intent copies the entire shared file
into the cache dir synchronously on the main thread before Flutter even
renders, which Android's watchdog eventually kills. Replaced with
hand-rolled handling in MainActivity.kt that only ever reads cheap Uri
metadata up front (ShareIntentService), so the destination picker always
appears instantly regardless of file size.

The destination picker (ShareUploadView) now mirrors the Files tab's own
controls/filters/listing instead of a bare folder list, and the actual
prepare+upload is handed off to ShareUploadService.kt, a real Android
foreground service with a single cancellable progress notification for
the whole batch - this survives the app being closed right after the
user confirms a destination, the same guarantee a file-manager app's own
upload notification gives.

Also fixes the "add another account" login page: it used to open in
url_launcher's bare inAppWebView, which has no close button and can hide
the status bar. LoginWebViewView is a real screen this app owns instead
(package:webview_flutter), with a normal AppBar/close button/safe area,
and can now auto-close itself on successful login.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

lib/services/upload_service.dart

@@ -0,0 +1,42 @@
+import 'dart:convert';
+import 'package:flutter/services.dart';
+import '../providers/server_provider.dart';
+import 'share_intent_service.dart';
+
+/// Hands a "Share to Noo" upload off to `ShareUploadService.kt`'s Android
+/// foreground service, which does the actual prepare+upload entirely on its
+/// own from there - including surviving the app being closed - with a
+/// single cancellable progress notification for the whole batch. See that
+/// service's doc comment for why this isn't done in Dart/Dio: a Dart
+/// isolate doesn't keep running once the Flutter engine is gone, only a
+/// real Android Service does.
+class UploadService {
+  static const _channel = MethodChannel('dev.ayushya.noo/upload_service');
+
+  /// Starts uploading [files] into the active account's current folder.
+  /// Fire-and-forget from Dart's perspective - once this returns, the
+  /// service owns the rest and reports progress/completion/cancellation
+  /// through its own notification, not back to the app.
+  static Future<void> startUpload(
+    ServerProvider provider,
+    List<SharedFileRef> files,
+  ) async {
+    final service = provider.service;
+    final authHeader = service?.authHeaders['Authorization'];
+    if (service == null || authHeader == null) {
+      throw Exception('Not logged in.');
+    }
+
+    final filesJson = jsonEncode(
+      files.map((f) => {'uri': f.uri, 'name': f.name, 'size': f.size}).toList(),
+    );
+
+    await _channel.invokeMethod('startUpload', {
+      'files': filesJson,
+      'serverUrl': provider.serverUrl,
+      'username': provider.username,
+      'authHeader': authHeader,
+      'remoteFolder': provider.currentFolderPath,
+    });
+  }
+}