Dedupe native transfer/notification code and split ServerProvider into focused controllers

Consolidates duplicated GET/PUT/notification-channel logic across
DownloadService/ShareUploadService/SyncEngine into shared Kotlin helpers,
gives upload/download real batch queueing instead of dropping a second
concurrent batch, and dedupes repeated Dart channel-argument boilerplate.

Replaces the 2300+ line ServerProvider god object with ten focused
ChangeNotifiers (SessionController, SettingsController, FilesController,
PhotosController, FavoritesController, TrashController, SharesController,
RecentController, SyncStatusController, PickController) plus ItemOperations,
a plain coordinator for cross-domain item mutations - fixing the coupling
where device-sync status, per-tab data, and global UI prefs all lived in
one object. Updates every view/widget call site accordingly and refreshes
the architecture/server/standards/styling docs to match.

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

lib/widgets/details/details_activity_tab.dart

@@ -2,7 +2,7 @@ import 'package:flutter/material.dart';
 import 'package:intl/intl.dart';
 import 'package:provider/provider.dart';
 import '../../models/nextcloud_item.dart';
-import '../../providers/server_provider.dart';
+import '../../providers/item_operations.dart';
 
 /// Per-file activity feed — same list-item look as the account-wide
 /// Activity tab (activity_view.dart), scoped to a single item.
@@ -21,8 +21,8 @@ class _DetailsActivityTabState extends State<DetailsActivityTab> {
   List<NextcloudActivity> _activities = [];
 
   Future<void> _load() async {
-    final provider = context.read<ServerProvider>();
-    final result = await provider.fetchFileActivity(widget.item);
+    final ops = context.read<ItemOperations>();
+    final result = await ops.fetchFileActivity(widget.item);
     if (!mounted) return;
     setState(() {
       _activities = result;