* [client] Fix exit node menu not refreshing on Windows
TrayOpenedCh is not implemented in the systray library on Windows,
so exit nodes were never refreshed after the initial connect. Combined
with the management sync not having populated routes yet when the
Connected status fires, this caused the exit node menu to remain empty
permanently after disconnect/reconnect cycles.
Add a background poller on Windows that refreshes exit nodes while
connected, with fast initial polling to catch routes from management
sync followed by a steady 10s interval. On macOS/Linux, TrayOpenedCh
continues to handle refreshes on each tray open.
Also fix a data race on connectClient assignment in the server's connect()
method and add nil checks in CleanState/DeleteState to prevent panics
when connectClient is nil.
* Remove unused exitNodeIDs
* Remove unused exitNodeState struct
