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.
xuwenwei
8a5ba65132