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/widgets/punch_status_banner.dart

@@ -0,0 +1,97 @@
+// punch_status_banner.dart
+//
+// Top-of-connection-page banner that shows whether the current session is a
+// P2P direct connection (NAT punch succeeded) or a relay fallback (NAT punch
+// failed, going through the user's 21117 server). User-facing copy makes the
+// implications clear:
+//
+//   - P2P: 绿色横幅 + ⚡ icon: "P2P 直连成功 — 走的是两台机器之间的网络，可放心传文件"
+//   - Relay: 橙色横幅 + ⚠ icon: "中继模式 — 视频/控制走服务器 21117 中转，请勿传大文件"
+//
+// The status comes from ConnectionType.direct, which the Rust core sets
+// when the connection is established (see src/client/io_loop.rs:184 —
+// set_connection_type(secure, direct, stream_type)).
+//
+// The banner is reactive: it subscribes to the Rx<String> so it updates
+// automatically if the connection type changes (e.g. after a re-connect).
+
+import 'package:flutter/material.dart';
+import 'package:get/get.dart';
+
+import '../../common/shared_state.dart';
+
+class PunchStatusBanner extends StatelessWidget {
+  final String peerId;
+  const PunchStatusBanner({super.key, required this.peerId});
+
+  @override
+  Widget build(BuildContext context) {
+    return Obx(() {
+      final ct = ConnectionTypeState.find(peerId);
+      if (!ct.isValid()) {
+        // Not connected yet — show nothing
+        return const SizedBox.shrink();
+      }
+      final isDirect = ct.direct.value == ConnectionType.strDirect;
+      if (isDirect) {
+        return _Banner(
+          color: const Color(0xFFE7F5EA), // light green
+          iconColor: const Color(0xFF1E8E3E),
+          icon: Icons.bolt,
+          text: 'P2P 直连成功 — 走的是两台机器之间的网络，可放心传文件',
+        );
+      } else {
+        return _Banner(
+          color: const Color(0xFFFFF3E0), // light orange
+          iconColor: const Color(0xFFE65100),
+          icon: Icons.warning_amber_rounded,
+          text: '中继模式 — 视频/控制走服务器 21117 中转，请勿传大文件',
+        );
+      }
+    });
+  }
+}
+
+class _Banner extends StatelessWidget {
+  final Color color;
+  final Color iconColor;
+  final IconData icon;
+  final String text;
+  const _Banner({
+    required this.color,
+    required this.iconColor,
+    required this.icon,
+    required this.text,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    return Material(
+      color: color,
+      elevation: 2,
+      child: SafeArea(
+        bottom: false,
+        child: Padding(
+          padding: const EdgeInsets.symmetric(horizontal: 12, vertical: 6),
+          child: Row(
+            mainAxisSize: MainAxisSize.min,
+            children: [
+              Icon(icon, color: iconColor, size: 18),
+              const SizedBox(width: 8),
+              Flexible(
+                child: Text(
+                  text,
+                  style: TextStyle(
+                    color: iconColor,
+                    fontSize: 13,
+                    fontWeight: FontWeight.w500,
+                  ),
+                ),
+              ),
+            ],
+          ),
+        ),
+      ),
+    );
+  }
+}