Add in-app copy/move, background downloads, and a Favorites tab

- WebDAV MOVE/COPY-backed copy/move for files and folders, with a
  destination picker (independent nav state so it doesn't disturb the
  Files tab's browsing position) and a conflict-resolution sheet
  (overwrite all / keep both / decide per item).
- Downloads now use the same foreground-service + notification approach
  as uploads, surviving app closure.
- Favorites is now its own tab (account-wide, via a WebDAV SEARCH query)
  instead of a Files-tab filter toggle, which couldn't represent
  favorited items outside the currently browsed folder correctly.
- Share sheet restyled to match the app's design language, plus a
  "share file directly" action via the OS share sheet; details sheet
  tab icons resized to match the bottom nav.

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

lib/services/download_service.dart

@@ -0,0 +1,52 @@
+import 'dart:convert';
+import 'package:flutter/services.dart';
+import '../models/nextcloud_item.dart';
+import '../providers/server_provider.dart';
+
+/// Hands a download off to `DownloadService.kt`'s Android foreground
+/// service - the download counterpart of `UploadService`/
+/// `ShareUploadService.kt` (see that pair's doc comments for why this
+/// isn't done in Dart/Dio: only a real Android Service survives the
+/// Flutter engine/Activity being gone, the same guarantee a real
+/// file-manager app's download notification gives you). Saves straight
+/// into the device's public Downloads folder with one cancellable
+/// progress notification for the whole batch, instead of the old
+/// download-to-app-cache-then-`file_saver`-prompt flow.
+class DownloadService {
+  static const _channel = MethodChannel('dev.ayushya.noo/download_service');
+
+  /// Starts downloading [items]. 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> startDownload(
+    ServerProvider provider,
+    List<NextcloudItem> items,
+  ) async {
+    final service = provider.service;
+    final authHeader = service?.authHeaders['Authorization'];
+    if (service == null || authHeader == null) {
+      throw Exception('Not logged in.');
+    }
+
+    final filesJson = jsonEncode(
+      items
+          .map(
+            (i) => {
+              'path': i.path,
+              'name': i.name,
+              'mimeType': i.mimeType,
+              'size': i.size,
+            },
+          )
+          .toList(),
+    );
+
+    await _channel.invokeMethod('startDownload', {
+      'files': filesJson,
+      'serverUrl': provider.serverUrl,
+      'username': provider.username,
+      'authHeader': authHeader,
+    });
+  }
+}