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/common/shared_state.dart

@@ -105,6 +105,25 @@ class ConnectionType {
         _direct.value != kInvalidValueStr &&
         _stream_type.value != kInvalidValueStr;
   }
+
+  /// Returns true if the connection is using P2P (direct) transport, i.e. NAT
+  /// punching succeeded. Returns false if going through the relay server.
+  /// Returns true (assume P2P) if the connection state isn't known yet, so the
+  /// UI doesn't briefly grey out before the connection establishes.
+  bool get isP2P {
+    if (!isValid()) return true;
+    return _direct.value == strDirect;
+  }
+}
+
+/// Convenience function: checks the P2P/relay state of a connection by peerId.
+/// Returns true (assume P2P) if state is unknown.
+bool isDirectConnection(String peerId) {
+  try {
+    return ConnectionTypeState.find(peerId).isP2P;
+  } catch (_) {
+    return true;
+  }
 }
 
 class ConnectionTypeState {