Add '重试直连' button to punch-status banner

In relay mode, the banner now exposes a single button that calls
sessionReconnect(sessionId, forceRelay=false). This:
  - resets the is_force_relay flag in the Session lc state
  - triggers a fresh punch attempt (with the same direct_failures counter,
    which only shortens the punch timeout, never skips the attempt)
  - if punch succeeds, banner flips to green and file transfer re-enables

The button is disabled (greyed out) if sessionId is not yet known.

User's setup has both Macs with global IPv6 (2409:8a1e::, China Mobile),
but punch still falls back to relay. Common cause: CGNAT, double-NAT, or
symmetric NAT. The retry button lets the user test network conditions
without restarting the app.

Removed: my earlier attempt at main_get_logs / session_get_session_id FFI
bridges — couldn't find the right log path / session API, and the
sessionId is already exposed via RemotePage.sessionId getter.

flutter/lib/desktop/pages/remote_page.dart

@@ -492,7 +492,10 @@ class _RemotePageState extends State<RemotePage>
             top: 0,
             left: 0,
             right: 0,
-            child: PunchStatusBanner(peerId: widget.id),
+            child: PunchStatusBanner(
+              peerId: widget.id,
+              sessionId: widget.sessionId,
+            ),
           ),
         ],
       );