[client] peer: re-arm the WireGuard watcher after a lazy wake (#7091)

## Describe your changes

The Conn struct is reused across lazy-connection deactivate/activate.
Close
cancels the WireGuard watcher (via wgWatcherCancel, and ctxCancel also
tears
down its context) but left conn.wgWatcher pointing at the stopped
instance.
enableWgWatcherIfNeeded skips while conn.wgWatcher is non-nil, so the
next Open
never started a fresh watcher: once a lazy connection had idled and
woken, the
peer ran with no watcher at all — no WireGuard handshake-timeout
detection and
none of the escalation that depends on it.

Clear conn.wgWatcher and conn.wgWatcherCancel in Close so the next Open
re-arms
a fresh watcher.

## Issue ticket number and link

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### Checklist
- [X] Is it a bug fix
- [ ] Is a typo/documentation fix
- [ ] Is a feature enhancement
- [ ] It is a refactor
- [ ] Created tests that fail without the change (if possible)
- [ ] I ran and tested this change locally — I did not rely on CI to
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## Documentation
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- [ ] I added/updated documentation for this change
- [X] Documentation is **not needed** for this change (explain why)

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<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Bug Fixes**
* Improved connection cleanup by fully releasing WireGuard watcher
resources when a connection closes.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

client/internal/peer/conn.go

@@ -307,6 +307,8 @@ func (conn *Conn) Close(signalToRemote bool) {
 
 	if conn.wgWatcherCancel != nil {
 		conn.wgWatcherCancel()
+		conn.wgWatcher = nil
+		conn.wgWatcherCancel = nil
 	}
 	conn.workerRelay.CloseConn()
 	if conn.workerICE != nil {