Add P2P/relay status banner + gate file-transfer menu on relay

Custom fork feature: surface NAT-punch result to the user so they know
whether the connection is going through their 21117 server (relay) or
directly between the two machines (P2P).

Three changes:

1. flutter/lib/common/shared_state.dart: add ConnectionType.isP2P getter
   (true = direct, false = relay) and a top-level isDirectConnection(peerId)
   helper that returns true when state is unknown (so UI doesn't briefly
   grey out before the connection establishes).

2. flutter/lib/desktop/widgets/punch_status_banner.dart (new): a small
   Positioned banner at the top of the connection page.
   - P2P success: green ⚡ banner: 'P2P 直连成功 — 走的是两台机器之间的网络，可放心传文件'
   - Relay fallback: orange ⚠ banner: '中继模式 — 视频/控制走服务器 21117 中转，请勿传大文件'
   The banner is reactive (Obx on ConnectionType.direct Rx<String>).

3. flutter/lib/common/widgets/toolbar.dart: wrap the 'Transfer file' and
   'TCP tunneling' menu items' onPressed with an isDirectConnection() guard.
   On relay mode, the menu still shows but the click is intercepted with a
   toast explaining why the action is blocked.

4. flutter/lib/desktop/pages/remote_page.dart: add the banner via
   Positioned(top: 0, left: 0, right: 0) at the end of bodyWidget's Stack.

Why this matters: user has 4 users on 5 Mbps server (114.55.133.123).
When NAT punch fails and the session goes through relay, file transfers
would saturate the server bandwidth. Showing the relay state + blocking
the file-transfer menu protects the server without requiring the user
to remember the technical detail.

flutter/lib/desktop/pages/remote_page.dart

@@ -22,6 +22,7 @@ import '../../utils/image.dart';
 import '../widgets/remote_toolbar.dart';
 import '../widgets/kb_layout_type_chooser.dart';
 import '../widgets/tabbar_widget.dart';
+import '../widgets/punch_status_banner.dart';
 
 import 'package:flutter_hbb/native/custom_cursor.dart'
     if (dart.library.html) 'package:flutter_hbb/web/custom_cursor.dart';
@@ -483,6 +484,16 @@ class _RemotePageState extends State<RemotePage>
               _ffi.ffiModel.pi.isSet.isFalse ? emptyOverlay() : Offstage(),
             ],
           ),
+          // Custom fork: punch-status banner — shows P2P/relay state at the top
+          // so the user knows whether the connection is going through their
+          // 21117 server (relay) or directly between the two machines (P2P).
+          // See flutter/lib/desktop/widgets/punch_status_banner.dart
+          Positioned(
+            top: 0,
+            left: 0,
+            right: 0,
+            child: PunchStatusBanner(peerId: widget.id),
+          ),
         ],
       );
     }